I give the commendation to the player I think did the best job. Along the run sometimes you see if someone does especially well. Sometimes there is more than one, or there is nothing special to remark and it's hard to choose.

Usually when you play as tank you notice more the healer's performance, but you can also notice a very good DPS, some runs are very different than others just because good DPS. If I didn't notice anything special, I usually commend the healer, it means he has done well his job if there have been no incidents.

As a healer you notice more how everybody is doing. If there is another healer that does nice I tend to commend him, you notice that easily, or the tank if he does well and no DPS has excelled.

When you go as DPS you notice more the other DPS performance, especially in 4 player content. You can see easier if he uses the DoT in a class that should use them, what rotations he does (if he plays a job you also play), if he uses the buffs, etc.