REGENA is the same as heal doing a heal action.
maxhp, doesn't work for healing (currently), it's a damage type, you can do MAXHP through the effects list on the action for healing though, you can cycle through TEMP, HP, MAXHP.
HEAL is a bonus of healing and not an action of healing. So if you have the effect "HEAL: 2d10" all out going heals will do an extra 2d10 healing.

