[Trexmaster]

From my understanding and having used the ballista myself, I think they mean that the AoE from things like Bursting Shot won't target things sitting on the borders of the target area.

For example:

XXX <-------- these tiles might not actually hit enemies on them
X X <--------- enemies in this space will get hit for sure
XXX <-------- these tiles might not actually hit enemies on them

I say 'might not' since I haven't extensively tested it but in all the cases I can recall if I put an enemy on the border of the AoE indicator they would not be hit, unlike how most AoEs display their range and will hit anything that is on the blue border squares.