There are several things we know about mana \ od. First is like a free energy which can be inhaled \ exhaled through the gate. The amount of second is decided at birth, and slowly vanishes until there is nothing left, BLAM, natural death. Mana amount one can manipulate (that's my wild guess) is related to od's pool - because Emilia is a half-elf, lives ridiculously long, has the biggest manapool of them all. Mana is not homogeneous and is split in six subcategories, two of them are pretty rare. They are water \ ground \ wind \ fire (ice as in absence of fire), yin (debuffs), yang (buffs). You weave spells by manipulating those elements, the harder the spell, the more complex the structure. You can actually invent new spells as long as you are proficient in combining shit. Important thing: spirits have od themselves, but their consciousness is kept active by mana, and they are able to weave spells themselves. Spirit user is like a person who uses automation to do their will instead of manual labor. Except, the automation have its own consciousness.