The ability that defines Daxos's toughness works in all zones, not just the battlefield.

Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).

Mana symbols in the text boxes of permanents you control don't count toward your devotion to any color.

If another creature you control enters the battlefield at the same time as Daxos, you gain 1 life for that creature. Similarly, if another creature you control dies at the same time as Daxos, you gain 1 life for that creature. However, in the unusual case that Daxos dies at the same time that another creature enters the battlefield or vice versa, you won't gain 1 life for that creature.

If you put an Aura on an opponent's permanent, you still control the Aura, and mana symbols in its mana cost count towards your devotion.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Daxos may become lethal if your devotion to white decreases during that turn.

Colorless and generic mana symbols (, , , , , and so on) in mana costs of permanents you control don't count toward your devotion to any color.

If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it's still on the battlefield at that time.

If you control a battle and an opponent is its protector, you still control the battle, and mana symbols in its mana cost count toward your devotion.