T5008 with negative amount

I have a client with T5008 slip with a negative amount in box 21 and box 20 is blank. my understanding from the client that this slip is

From futures that don’t have a traditional book value since they are marked-to-market. The gains/losses are already reflected on the T5008.

Is this still entered in schedule 3 to have a capital loss or entered in line 22100? Your replies greatly appreciated.

Likely means a capital loss, from what you describe. I can’t imagine any way it would belong on line 22100.