Some of those are passive like Manabreak (Antimage). Orb Effect can be provided by some skills in the game aswell. That also mean that on a ranged hero you can have Skadi + Lifesteal (Mask of death, Mask of madness, Satanic) + Mjollnir/Maelstrom and a Sange and Yasha and you would benefit from all passive debuff. Basically you can lifesteal, corrupt (desolator) etc. When you do not proc it and if you have another orb effect in your inventory, it will be applied. The Orb Effect chain lightning from Mjollnir (and Maelstrom) depend on if you proc it or not (25% chance).
So only a lifesteal, no Skadi + desolator for example.
As of patch 7.07 : all Orb Effect (Unique Attack Modifiers) have been removed.