A picture you can hold: a gate with a line drawn across it. Whoever or whatever stands at that gate decides what may pass and how far it may go. A runner who gets through the gate is a qualifier. The race that holds the gate is also a qualifier. And a small word such as very or const stands beside a bigger word and draws the same kind of line, saying how far that word is allowed to reach. In every use, the word names the thing that sets the condition — not the prize on the other side.
Wiktionary's entry gives the word as qualify + -er: the one that qualifies. That fits the modern uses, where the word names the person, event or word that meets or sets a condition.
來源把這個字拆成 qualify 加上 -er，也就是「做 qualify 這件事的人或東西」。qualify 本身有兩條線：一條是「達到標準」，一條是「加上條件、限定住」。今天 qualifier 的四個用法，剛好分別長在這兩條線上。
Wiktionary (kaikki extract) entry for 'qualifier', etymology_text: 'From qualify + -er.'
Two branches
過關的人，和守關的那場比賽
這一組走的是 qualify 的第一條線：達到標準。門口的兩邊各拿到同一個名字——過門的人是 qualifier，那道門本身也是 qualifier。
noun · B2
Someone who has reached the standard needed to enter a competition or to go into the next round, usually by beating other people in an earlier race, match or test.
晉級者
替別的東西設下條件的那個字
這一組走的是 qualify 的第二條線：加上條件、把範圍限定住。門還是那道門，只是這次被擋住的不是人，而是一個字能涵蓋多少。
noun · C1
A word or phrase, often an adjective or an adverb, that is put next to another word or phrase in order to limit its meaning or add something to it.
修飾語