1. "Extended review" means "under review." It is a term loosely used by some representatives and could be anything, security checks, officer review, or in another word, just normal processing.
2. "Extended review" means "final review." It is referring to the final stage of the processing after security checks have been cleared and the case is ready to be adjudicated.
3. "Extended review" means "external review." It means USCIS is awaiting information from another government agency.
Before USCIS cares to offer an official explanation, we tend to believe theory #1 is the case.