During my multiplayer games I've been getting ether an 'assist' or a 'kill' when a target I was hurting died, and a number next to it. I'm guessing that the numbers represent the amount of xp I got for that kill. What I don't understand is when a kill is a 'kill' and when just an 'assist'. It happened to me that I've got a kill on the same enemy type that I got an assist for, but with less xp. How is a kill/assist determined?
Kill: You were the one who scored the final hit on the enemy to put them down.
Assist: You damaged the enemy but didn't land the last hit which killed him.
It is completely possible for a person who assisted in killing an enemy to get more points than the person who eventually kills that enemy. It all depends on how much each person personally contributed to the enemy's eventual demise.