As and when you create a service category (by tapping on Hamburger icon, then Queue settings in the drawer menu and then Manage Service categories), you can add questions specific to that service category. The question that you add, can be marked as mandatory or you can keep it as optional. Whenever customers check-in to your business, select a particular service category, they will have to answer the question if you mark it has mandatory.
However adding questions to your service category is not mandatory. You can simply add a service category and do not add questions to it.
Once you add question(s) to a service category, you will be able to delete the question anytime and also change the sequence of the questions.