At the expiration of the contract you are trading, all positions get closed out and settled. Different exchanges use different anchors to settle against. To avoid manipulation, most of them use an index. OKCoin uses a custom 6-exchange index (3 chinese, 3 Western), while BitMEX uses a third-party XBX Index to settle all of its contracts. This minimizes the risk of any manipulation if contracts settled on a single exchange's price, but it also makes it more difficult to manage your hedges since there's no way to "buy" or "sell" an index. No system is perfect, and each one offers different unique pro's and con's, just like your selection of contract type and length. So depending on your case you may want something different than someone else.
CryptoFacilities has a daily rebalancing, while BitMEX has weekly rebalancing scheduled for all contract lengths and the Chinese futures exchanges have a weekly settlement procedure. It's important to note that on all of these exchanges, all the futures and forwards are non-deliverable and have expiration at various times. Despite this, OKCoin uses "delivery" as their term for expiration settlement of a contract, and "settlement" for the weekly rebalancing of the Pnl for higher length contracts. Since we are stuck with this unprofessional Chinese exchange being the dominant market player in futures, we have to focus on their use of the terminology, but BitMEX and CryptoFacilities have very impressive and professional setups that use well documented procedures. Always read the documentation of any exchange you're using and the contract specifications of the individual product you're using.
On OKCoin you have three main kinds: