Kills that you have not redeemed. Which is why its sort of difficult to leave- do I save up these proofs to get out, or do I use a few so I can get gear and not die?

And then as you'll soon see, weaker/common monsters like shamblers(literally zombie corpses of dead Expeditionary soldiers and colonists) are not counted as kills.

Also, even if the squad kills a monster, only one person can claim a proof of kill, which generally becomes a point of contention.