Items That Self Dmg Classic Wow

Hello there. Information of this seems sparse. I'm looking at fine details, not just matter-of-fact.
I'm wondering how damage/healing items work.
Specifically:
What does this do: [up to 40 damage / healing] ?
What does this do: [+40 damage]?
What does this do: [+40 healing]?

I believe these are based on cast times for the maximum effect ... but 'up to X damage'? does that mean 1dX RNG? Up to 40 damage is 1d40 damage? AVG~20?
Cast time matters. An instant cast, as a wand, cannot expect those rewards. But what about wands with +damage or +healing? Healing wands are very sought after.
Is there a difference between [up to] damage vs +damage .. or [up to] healing vs direct +healing
Anyways, an explanation of +damage +healing items would be appreciated. It's probably simple, yet elusive to me. Thanks!
Items That Self Dmg Classic Wow

Items That Self Dmg Classic Wow Release

Items that self dmg classic wow release

Items That Self Dmg Classic Wow Server

But the dmg alone is not the main thing that makes this trinket worth it. It's what each attack can do. In the past few hours i've been using this in Bg's i've gotten:-Fireball which hits for 800-900 non-crit and about 1500 crit. I haven't been able to notice if it leaves a dmg over time on the enemy yet.