This is a feature in margin trading. By "claiming a position," you use your own assets to close the position and basically turn a margin trade into a spot trade.
In Margin Trading, Positions block, next to each open position there is a "Claim" button. Long positions can be claimed with quote currency assets, and short positions with base currency assets.
For example, a long position for 3 BTC at price $500 will be claimed with $500*3 = $1500. Similarly, a short position for 3 BTC will be claimed with 3 BTC.