Pay/Collect

What Does Pay/Collect Mean?
An abbreviated reference to the payment or collection of funds (after futures positions have been marked to market) between clearing members and their respective clearing houses.

Investopedia explains Pay/Collect
Typically, futures positions are marked to market every evening after exchanges are closed for trading. As futures trading is a zero-sum game, in marking to market, one side of the futures position will be in a deficit position while the other in a surplus. This imbalance is offset through the pay/collect transactions executed by the brokers to their clearing organizations.