Affinity

Affinity is a per-element measure of how attuned a character is to fire, water, air, or earth. It's an optional byproduct some abilities produce — not every ability, or even every Skill, interacts with it at all (Thieving, for instance, produces none and needs none).

The core mechanic — how affinity grows, and how it makes elemental use cheaper/stronger, including synergy between allied elements and dissonance between opposed ones — is defined on Pneuma (see Pneuma's Three Laws and its Affinity section). That page covers affinity as it applies to spellcasting and physical abilities powered by pneuma.

Scope: global per element

Affinity is tracked globally per element, not per skill. Any elementally-tagged ability, in any skill, feeds the same pool: smelting at the forge and casting fire spells both build the same Fire affinity, and both benefit from it. A blacksmith who spends years at the forge will cast a hotter fireball than one who's never touched fire, without ever having cast a spell.

This generalizes Pneuma's Law III ("affinity grows by spending that medium — casting or exertion") beyond spellcasting to any ability tagged with an element, regardless of which skill it belongs to.