qualifier token a chosen word

which layer

also called electrode layer · tip, core or inlay

WHICH LAYER of one electrode a material or a diameter is about. It exists because a maker publishes two different metals for the same electrode: NGK gives the BUHW (2622) an "Electrode Tip Material: Tungsten" and an "Electrode Core Material: Copper" on one centre electrode, and the Ruthenium HX a ruthenium fine wire over a copper core. With `electrode_side` alone those two statements are unordered, and the pool would say only that the centre electrode is somehow copper and tungsten. NOT REQUIRED, deliberately: a maker who publishes just "iridium centre electrode" has stated no layer, and forcing one would invent a fact the sheet does not carry.

Which layer is answered with one word chosen from the electrode layers list — the same word shared across every product that uses it.

It is a qualifier of electrode tip diameter — it has meaning only inside a electrode tip diameter statement, one per statement, never on its own. It belongs with electrode tip diameter to measurement — its measurable, physical facts.

It is answered inside every electrode tip diameter statement, not as a standalone question on a kind.

answered with: ◇token — a token — a word chosen from a fixed vocabulary, shared and filterable across products.
value domain: Electrode layers — the vocabulary its answers are drawn from.
qualifier of: electrode tip diameter — one per statement — it has meaning only inside that host's statement.

Answered inside every electrode tip diameter statement — not a standalone slot on any kind.