First, make clear the principle of futures trading: no matter what kind of trading, we must abide by such a formula: long position+short position = short position+short position. In your example, A sells open positions 1 lot, B sells open positions 10 lot, C buys 5 open positions, and then opens 5 positions. Because C only bought 5 lots and traded 5 lots, the futures trading volume is calculated by bilateral spot trading 10 lots, which is multi-flat, because one of them closed the position, regardless of whether the two sides opened the position. Now let's talk about the position difference, because a flat 1 hand, b open 4 hands, c open 5 hands, the position difference is 0, empty open 4+ open more 4= position difference +8. This is too simplistic, you should understand, and you don't know how to keep asking.