In the case of PH 315 and PH 400 - the lecture and lab classes are sometimes linked together. For example, if you register for the lecture PH 400-01, you will need to register for a lab/activity section that corresponds to that lecture, usually indicated by the number of the lecture, in this example -01. So if you register for PH 400-01 (Lecture) you must register for a lab/activity section such as PH 400-1A, PH 400-1B, etc., and not PH 400-2A, PH 400-2B, etc., which would be linked to the lecture PH 400-02.
It sometimes happens that the system will allow you to register for a mis-matched lecture and activity section, but you will be stuck on the waitlist until you choose a lecture and lab/activity section that link together.