The distinction is in if the healing effect is a 'tick' or directly attributable to the spell. Whenever someone is healed, ask this question:
Does the spell that caused the heal to happen cast in the same frame/game logic tick as the heal itself?
Iff yes, then this is a direct heal.
Iff no, and there are multiple instances of time in which heals happen, it is periodic. This includes both DoT and channelled effects.
Counts as periodic
- Area effects with 'ticks'. (Healing Rain, shaman).
- DoT spells (Rejuvenation, druid)
- Drain life (Warlock)
Not actually periodic
- Area heals (Healing Circle, priest)
- Direct heals (Chain heal, shaman).
- Combined DoT/Direct spells (Riptide, shaman)1.
- Healthstone (Warlock)
1: Only the initial cast will remove a Grievous stack, not every tick.
For some spells/effects, it's unclear of whether they would count, and likely up to the developer. As it's unlikely every corner case was covered, there may be some surprising inconsistent details. Consider effects like:
- Living seed. (Adds a separate instance of heal not from a spell.)
- Priest' Trail of light (again, separate instance)
- Cloudburst totem
It's likely up to the specific code implementation of each spell whether or not Grievous would count them as periodic or not. Someone would have to make a giant list of spells and test each one if they count.
It's quite likely most of these would not count as periodic with a few exceptions.