Search/Register For Sections (XWCT) and Search For Sections (XWSE) will show the number of available seats for waitlisted sections as 0 even though there may technically be open seats in the section.
COURSE.SEC.PENDING is a space holder file to reserve a seat for a student while the registration process is finishing. If someone is registering for a class, the one field in the file, CSP.RESERVED.SEATS, gets set to 1 (or 2 if two students are registering, etc.). The number in the CSP.RESERVED.SEATS field is added to the "used" capacity for the section. This is to prevent overfilling the class. So if there are 19 students signed up for a class with a capacity of 20, and 2 more are trying to get in at the same time, they can't due to the COURSE.SEC.PENDING file. This would take the used capacity to 21 which is over the limit.
There is an issue where, if someone starts a registration session (through UI or WA) but doesn't finish out and cancels the process in the middle, the CSP.RESERVED.SEATS doesn't get set back to 0.
When students and advisors are restricting their search for only open sections for registration. This will cause a section to appear to be closed when it is in fact open. The effects both the CAS search for sections results (i35TW12B) and the CAS search and register for sections results (XWSTS12B).
There are a couple of solutions that will correct this problem.
Quiet System Solution
Ellucian suggests doing a CLEAR.FILE of COURSE.SEC.PENDING to correct the records. This should be done on a quiet system because if students are currently registering for a class (thus CSP.RESERVED.SEATS is at 1, then set to 0 by the clearfile) the section can be overbooked because the CSP.RESERVED.SEATS was correctly set to 1 in this case. Instead of clearing the entire file, you can do SELECT COURSE.SEC.PENDING WITH CSP.RESERVED.SEATS GT "0".
Live System -- One Record at a Time
If you'd like to take care of one record at a time, you can do this on a live system. You can find the COURSE.SEC.PENDING.ID which is the same ID as the COURSE.SECTIONS.ID. Edit that COURSE.SEC.PENDING record. Replace the number in field 1 with a 0.