Spirit and reservation in Path of Exile 2
Where Spirit comes from in PoE2, what reserves it, how reservation efficiency works, and how to find room for one more aura, buff or meta gem.
What Spirit is for
Spirit is the resource persistent skills use. Buffs, auras, heralds, meta gems like 
Cast on Critical, minions and companions all reserve some of it for as long as they're active. When your Spirit runs out, you can't turn on anything else.
Unlike mana, Spirit doesn't regenerate or get spent - it's a budget. The question is always what to fit into it.
Where Spirit comes from
- The campaign: three quests give Spirit (+30, +30 and +40). Make sure you've done them
- Sceptres: the main weapon source, with base Spirit plus 'increased Spirit' mods that multiply it
- Amulets and body armour can roll '+ to Spirit'
- A few uniques and runes add Spirit, sometimes in slots that normally can't have it
- A few passives and ascendancy nodes change your Spirit, usually with a trade-off
What reserves Spirit
Most persistent skills reserve a flat amount. Many buffs cost 30 Spirit; meta gems like Cast on Critical and 
Cast on Elemental Ailment cost more. Totems with 
Ancestral Bond reserve Spirit each, so Spirit decides how many totems you can run.
Companions are different: a tamed beast reserves a percentage of your total Spirit, which depends on the beast. That makes them expensive on characters with a lot of Spirit - the more Spirit you have, the more the beast takes.
Reservation efficiency
Reservation efficiency makes everything cost less. It rolls on helmets, comes from some runes and uniques and a handful of passives, and some gems can get it from quality. It applies to every reserving skill at once, so on a build with several Spirit skills a few percent can free a surprising amount.
Efficiency divides costs rather than subtracting from them, so the gain from each point gets slightly smaller as you stack more.
Finding room for one more skill
Before buying new gear, check the cheap options:
- Lower-value Spirit skills you could drop, especially ones that do little for your damage or defence
- Support gems in your Spirit skills' links that add reservation for a small benefit
- Reservation efficiency on your next helmet or belt
- Gem quality, where the gem's quality gives efficiency
Then look at the gap. If you're five Spirit short, a small efficiency roll fixes it; if you're fifty short, you're looking at a better sceptre or giving something up.
PoE MVP's report shows your free Spirit and which buffs and auras fit into it, measured in Path of Building.