What is the "Alternate Characters Key" in Gnome Settings under Keyboard Shortcuts?

As @fmuellner said (and is somewhat explained in the dialogue) it’s AltGr

Alternative Characters Key
Hold down and type to enter different characters

The alternate characters key can be used to enter additional characters. These are sometimes printed as a third-option on your keyboard.

Whilst your keyboard may only have 2 characters per key they can actually produce 4

Modifier Level Key Result
[None] 1 A a
Shift 2 A A
AltGr 3 A æ
AltGr+Shift 4 A Æ

The full mappings for my keyboard (See Settings ⟶ Region & Language ⟶ Input Sources ⟶ “Eye” Button)
image

Note on a UK keyboard we do have keys with 3 printed symbols (¬ ` | and 4 $ €)
image

We also do not have a second Alt key, instead having an AltGr key - otherwise we couldn’t use €
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So whilst your keyboard may not have a printed AltGr I suggest you just set the “Alternative Characters Key” to be “Right Alt” - highly unlikely your using that key for anything anyway

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