ability_affinity

An ability's element affinity does not live in the ability definition; it is a definition of its own in this data pack registry. The reason is simple: which elements one ability belongs to is a rule of the world, not part of the ability's shape - the same saturn:ability entry can be reused by any mod, while "it leans towards fire" only holds in this world.

File and Id

• The registry id is mxt:ability_affinity, and the file goes under data/<namespace>/mxt/ability_affinity/<path>.json.
• One file is one definition and the entry id is <namespace>:<path>; nothing else references that id, so any name works.

// data/example/mxt/ability_affinity/qi_bolt.json
{
  "abilities": ["example:qi_bolt"],
  "elements": ["example:fire", "#example:basic"],
  "mode": "average"
}

Fields

Field	Type	Default	What it does
abilities	List of ability ids or #ability tags	Required	Which abilities this affinity covers; not empty.
elements	List of element ids or #element tags	Required	The elements the ability leans towards; an empty list means "this ability is not element-bound".
mode	average / max	average	How several matching roots combine: average takes the mean, max takes the best one.

First match answers. One ability can be hit by several definitions (another pack names it too, or one writes a concrete id while another writes a tag). The first entry in registry order wins and the rest are not read - so to make a concrete id win over a tag, put that entry first instead of relying on a priority field.

What It Does

When an ability is named here and its elements is not empty, it gets two things:

1. A cast gate: a holder whose spirit roots match none of them is refused, with a permission failure.
2. A damage multiplier: computed from the matching roots' element_ability_modifier, averaged or best per mode. It is folded together with the progression chain's multiplier into the formula variable damage_multiplier, which layer one of the damage pipeline multiplies into the damage this cast deals.

So a damage formula must not write any affinity multiplier by hand, and there is no element_modifier variable any more. To scale something that is not damage (costs, durations) by affinity there is currently no variable to read.

The wheel tooltip lists those elements under the ability; with no definition, or with an empty elements, neither line is drawn.