Perpetual Futures are a special type of Futures contract that have no expiration date and have an autorolling feature every fourhours.
To see Fixed Maturity Contract specifications please visit the other page here.
Below is a detailed table of characteristics, followed by multiple examples to demonstrate the mechanics of the Contract:
Perpetual Contract Specifications

BitcoinDollar Futures 
EtherDollar Futures 
LitecoinDollar Futures 
BitcoinCashDollar Futures 
RippleDollar Futures 
RippleBitcoin Futures 

Contract Symbol 
PI_XBTUSD 
PI_ETHUSD 
PI_LTCUSD 
PI_BCHUSD 
PI_XRPUSD 
PV_XRPXBT 

Instrument Type 
Inverse Perpetual Futures 
Vanilla Perpetual Futures 

AutoRoll Period 
Every 4Hours on 12 UTC, 16 UTC, 20 UTC, 24 UTC, 4 UTC, 8 UTC 

Ratesetting Calculation Window 
Rate for next period is calculated over current 4hour period (e.g., rate for 1216 UTC period is calculated in window between 812 UTC 

Funding Rate 
Between start and end of Ratesetting Period the Funding Rate is computed as the timeweighted average premium, and standardized to a perhour basis. Permissible range per 1 hours: [0.05%, +0.05%] (i.e., 40 basis point magnitude for 8hour realisation period) 

Payout Frequency 
Continuously based on Funding Rate set at the end of the prior Funding Period. Positions will immediately and continuously receive or send funding while open in the perpetual contracts. The funding accumulates as UPL and settles every four hours at end of Funding Period, or when user changes net open position (whichever occurs first). 

Funding Rate Multiplier 
n = 8 This is the coefficient used in the calculation of the funding rate. A value of 1/n means that, ceteris paribus, it will take n hours to realize the Average Premium. Example: if the Average Premium is 0.32% for the four hour period, then Funding Rate is equal to 0.04%, meaning that over the course of 8 hours, this 0.32% total will be realised. 

Funding Rate Calculation 
In a given 4hour Funding Period, Premium values calculated from minutely perpetual contract prices (240 observations) are recorded versus the Real Time Platform Ticker. The Average Premium is calculated as the average of the mid 120 observations recorded from above. Finally, this value is weighted by the Funding Rate Multiplier. If Average Premium > 0 then for the 4 hour period, those in Long positions will continuously pay out to Short positions, which pushes the price closer to Index. If Average Premium < 0 then for the 4 hour period, those in Short positions will continuously pay out to Long positions, which pushes the price closer to Index. 

Contract Size 
1 USD 
1 USD 
1 USD 
1 USD 
1 USD 
1 XRP 

Trading Hours 
24 hours/day, 7 days/week, 365 days/year (excluding maintenance) 

Price Quotation 
US Dollars per 1 bitcoin 
US Dollars per 1 Ether 
US Dollars per 1 Litecoin 
US Dollars per 1 BitcoinCash 
US Dollars per 1 Ripple XRP 
Bitcoin per 1 Ripple XRP 

Tick Size 
1 USD 
0.1 USD 
0.01 USD 
0.1 USD 
0.0001 USD 
0.00000001 BTC 

P&L Settlement Method 
Cashsettled in XBT 
Cashsettled in ETH 
Cashsettled in LTC 
Cashsettled in BCH 
Cashsettled in XRP 
Cashsettled in XBT 

Fee Structure 
0.075% Taker 0.030% Maker (Instant Rebate) 

Contract Expiration 
The perpetual futures are nonexpiring, which means that positions in the contract are never “expired” or “matured”, however there is a settlement process every four hours that applies funding to anchor the spot value to the Index. See Last Trading for more information. 

Settlement time 
Every 4 Hours on 12 UTC, 16 UTC, 20 UTC, 24 UTC, 4 UTC, 8 UTC: the accumulated unrealised funding is settled and new rate set based on TWAP premium to index in prior rate period. 

Last Trading 
This contract remains trading perpetually an would only expire in emergency situation if Crypto Facilities deems necessary to settle/expire contract. This would only occur in exigent circumstances if the Market Risk Committee deemed it necessary to facilitate a fair and orderly market. 

First Trading 
August 31, 2018 

Initial Margin 
Minimum 2% 

Maintenance Margin 
Half of IM 

Position Limits 
See Margin Schedule 

Mark Price 
Mid price of Order Book bounded by a range defined by CME CF Bitcoin Real Time Index (BRTI) with antimanipulation coefficient 
Mid price of Order Book bounded by a range defined by CME CF Ether Real Time Index with antimanipulation coefficient 
Mid price of Order Book bounded by a range defined by LTC:USD Spot Rate with antimanipulation coefficient 
Mid price of Order Book bounded by a range defined by BCH:USD Spot Rate with antimanipulation coefficient 
Mid price of Order Book bounded by a range defined by XRP:USD Spot Rate with antimanipulation coefficient 
Mid price of Order Book bounded by a range defined by XRP:XBT Spot Rate with antimanipulation coefficient 

Real Time Platform Ticker 
IN_XBTUSD 
IN_ETHUSD 
IN_LTCUSD 
IN_BCHUSD 
IN_XRPUSD 
IN_XRPXBT 

Margin & Settlement Currency 
XBT 
ETH 
LTC 
BCH 
XRP 
XBT 
Note on Funding Rate
The funding rate for a given perpetual contract is represented in two different ways:
 absolute rate: The amount of funding an account will receive by maintaining a 1 contract unit short position for 1 hour. This is more useful for account log purposes.
 relative rate: The absolute funding rate relative to the spot price at the time of funding rate calculation. This is an intermediate value in the calculation of the absolute funding rate, and is the number we display in the front end (as a %) as the 'funding rate'.
Examples
In order to get a complete understanding the rate dynamics of the Perpetual Contract, we present examples to demonstrate the key features:
Example Funding Rate 1: (8hour rate realisation length)
Assume time is 12 UTC and that price of BTC is $7,000 (via realtime index) and the Perpetual trades at $7,010 the whole time until 16 UTC. The average premium is computed as 0.1428% for the 4hour period ($10/$7,000). This leads to a funding rate of 0.1428 / 8 = 0.01785% per hour.
Now assume that you are in a short position of 100,000 Contracts. If you hold this position from 16 to 24 UTC, and the premium in 1620 UTC period remains as 0.1428%, then you will earn interest of $142 for the eighthour period ($100,000*0.001428) in BTC terms, so $142/$7,000 = 0.0204 XBT.
Example Funding Rate 2: (Maximum rate)
Assume time is 12 UTC and that price of BTC is $7,000 and the Perpetual trades at $7,100 until 16 UTC. The average premium is computed as 1.428% for the 4hour period. This leads to a funding rate of 1.428 / 8 = 0.1785% per hour.
The maximum funding rate per hour in any given period is 0.05%. The minimum is 0.05%.
As a result. this 0.1785% hourly rate is floored to 0.05% per hour, so that the maximum 8hour realisation will not exceed 0.40%.
Note that there is no "dampening" of rates done in this model: if a fourhour computed rate is near 0, then it will stay pay out non0 value even if it is de minimis.
Example Funding Rate 3: (Variable interperiod rate)
Assume time is 14 UTC and you enter a position long 200,000 Contracts on XBTUSD at $7,000. Assume that the funding rate for the fourhour period (1216 UTC) is set as 0.04% per hour.
At 16 UTC, after you have held this position for two full hours, you will have earned $80 per hour (0.0004*$200,000). This is $80*2=$160 USD equivalent, or $160/$7,000 = 0.0228 XBT, which credits continuously throughout the two hour period you hold it.
However, during this period , the price was at a premium and so the new fourhour rate set for 16  20 UTC is 0.04% per hour. After two hours of holding the position, you have paid 0.0228 XBT and you close at 18 UTC, two hours later.
Your funding for the four hour period you held the position is thus 0.0228 XBT for the last two hours of first period then 0.0228 XBT for the first two hours of the second period and your net flows are 0 for this.
Example Funding Rate 4: (Fixed intraperiod rate)
Assume time is 14 UTC and you enter a position long 500,000 Contracts on XBTUSD with realtime index at $7,000. Assume that the funding rate for the fourhour period (1216 UTC) is set as 0.033% per hour.
At 16 UTC, after you have held this position for two full hours, you will have paid $165 per hour (500,000*0.00033). For two hours this costs $165*2=$330 USD equivalent, or $330/$7,000 = 0.04714 XBT, which debits from your position continuously throughout the two hour period you hold it, in the form of Unrealised Profit/Loss (UPL).
Example Funding Rate 5: (Settlement of unrealised funding)
Assume its 12 UTC you are in a 250,000 Contract long position on XBTUSD with realtime index at $7,000 and the rate in the funding period is 0.05%. This earns you funding of:
 $125/$7,000=0.01785 XBT per hour
 0.0002976 XBT per minute
 0.00000496 XBT per second
 0.00000000496 XBT per millisecond
This credits and debits every millisecond to every user with an open position. It credits first as "UPL" which is "unrealised" profit and loss, but you have the funds available right away to use in further positions or transfer into you Cash account.
The funding accumulates as UPL and is booked into your account log and realised when one of the following events occur:
1. You adjust your open position up or down by any amount
2. You hold until the end of the Funding Period, at which point it is booked (occurs every four hours)