When you enable the Reduce flag, the order can only reduce the size of your position.

If you submit an order with the Reduce flag, and it would increase the size of your position upon being matched, the order is automatically cancelled by the Everstrike Matching Engine.

This can be helpful if the intent of the order is to (partially) close your position.

Orders with the Reduce flag will also never turn a long position into a short position (or vice versa). This could otherwise happen if you are long by $200 and get filled on a sell order that is valued at $300. If the sell order did not have the Reduce flag enabled, your long position would be turned into a $100 short position. If the order did have the Reduce flag enabled, the order would be cancelled instead, and you would still be long by $200.