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)
Note on a UK keyboard we do have keys with 3 printed symbols (¬ ` | and 4 $ €)
We also do not have a second Alt key, instead having an AltGr key - otherwise we couldn’t use €
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