What can a sub see in the faculty portal when they are assigned to a teacher's class for the day?

When you assign a substitute to a teacher's classes (if you have the Employment module, via their faculty attendance detail record, and if you don't, then by manually adding them to the Permissions tab of the class record), the sub has access to the classes in the Faculty portal as well as class assignments and the gradebook. In other words, the substitute has full teacher access in the portal to the classes they are a sub for on the day(s) they are a sub. Ensure that the sub has a Veracross user account and a Faculty security role otherwise they won't be able to login to the portal.