When a reviewer sees a submission that is beyond their immediate field of technical expertise or understanding, they usually just leave it be. Rather than trying to do quickie research on a topic they know little about, the reviewer leaves it for someone more knowledgeable on the subject.
However, when a technology falls between or beyond the immediate fields of technical expertise of the entire reviewer collective, the submission is often left idle for very long periods of time and is forgotten until bumped. Even after bumps, such submissions are usually left to rot longer.
This tends to happen with more scientifically advanced-sounding submissions. Tech staff will sometimes reply to these submissions with minor suggestions, but that is irregular.
Submissions that could drastically alter the roleplay, and are therefore controversial, might deliberately be taken to the Submissions Office for private discussion among tech staff.
It used to be these submissions were collectively ignored whether they were controversial or not. Commonly they were race-related submissions of technology, such as a weapon for a certain race. The Submissions Office now handles these, so Type 2 is less frequently seen.
Submissions that are so huge or numerous (subsystems, etc) that moderators cannot find time to review them.