In Pathfinder 2e, what is the key ability for an archetype’s Class DC?

Taking certain archetype dedication feats, you become trained in the archetype class’s class DC, as for example in Monk Dedication:

You become trained in monk class DC.

(https://2e.aonprd.com/Archetypes.aspx?ID=8)

The dedication feat does not specify a key ability for the new class DC. Conversely, a key spellcasting ability is specified for spellcasters (e.g. Sorcerer Dedication):

Your key spellcasting ability for sorcerer archetype spells is Charisma,

( https://2e.aonprd.com/Archetypes.aspx?ID=11 )

The rules state the class DC is based on the class’s key ability’s modifier:

This DC equals 10 plus their proficiency bonus for their class DC (+3 for most 1st-level characters) plus the modifier for the class’s key ability score.

(https://2e.aonprd.com/Rules.aspx?ID=102)

For several classes, the key ability is not clearly defined: Champion, Fighter and Monk all have “STRENGTH or DEXTERITY”, and the key ability for Rogues depends on their racket, which you do not choose in an archetype dedication.

So, what ability is used for calculating the new Class DC? Do I:

  • choose freely from the available options (using Dexterity for Rogue Dedication because it has no racket)?
  • use the key ability from my main class?
  • or don’t apply an ability modifier at all because no ability is specified?