For clarification in the below image the purple ellipse is the unit's hitvolume. The grey sphere is the aimsphere. The aimsphere does many things that are probably better off being split into separate tags. I haven't tested it much but it at least controls how close a unit is considered to be when aoe damage occurs.
Anyway, the important function at the moment is how it relates to aiming (which incidentally is why I decided to call it an aimsphere). Units with avoidFriendly will avoid both the aimsphere and the hitvolume of an allied unit when aiming. Projectiles only collide with the hitvolume so this can create situations where units can be blocked from firing at an enemy while the enemy is able to fire at them.
Don't ask me why the aimsphere does this, it doesn't make sense to me. I'd just like to know how to make them small enough to not be a hassle. Also I suspect there is a similar thing for features with custom collision volumes. If they have them it would cause the aiming issues I have seen from units with avoidFeature.