Hi Michael - need more background detail on your question, e.g. are you using services created in Digital Workplace Catalog, or SRM related/imported services? what version/platform are you on? etc...
Like mentioned by Emmanuel Galia we need some more information but if you are using DWP Advanced and building your services in the broker this applies.
Depending on version it differs if i dont recall incorrectly. Atleast on 18.11 and above you can have required questions in conditional branches which makes them mandantory only on that condition:
In the example above i fetch an attribute (hidden from user) that indicates if justification is needed or not for that specific request. This could just as well be a condition based on earlier manual answers in the questionnaire.
I think i had to do a workaround on earlier versions though but you dont mention what version you are on so i hope this answers your question.