How do abilities stack?

In my map, I have two towers that I am concerned about conflicting. One simply has a modified Acid Bomb ability. The other, has a custom shockwave ability which casts a single target (no AoE) Acid Bomb on each unit hit by it.

Now, I am not using the default Acid Bomb buff. Both towers have custom buffs, different from each other.

If both towers are in play, and a creep is hit by both of them, what will happen?

Buff spells based on the same base spell will not stack even if they use different buffs. Auras, however, will.

While we're on the topic, how does stacking actually work? I mean, how do you make an ability that does or doesn't stack?

What exactly are "buff spells" ?

Spells that leave a buff? Like inner fire, soul burn, bloodlust, unholy frenzy, acid bomb, silence, drunken haze, invisibility, entangle, sleep, polymorph, shadow strike, roar, lightning shield, anti-magic shell, doom... That sort of stuff.

You know, as opposed to spells that don't leave a buff, like death coil, chain lightning, dispel magic, holy light, mana burn... well, we can't really talk about stacking in this case, since they're all instant spells.