Flags is the system used to describe the state of completion of a page. Each question page is composed of three main elements: a question statement (Q), one or more hints (H), one or more solutions (S). Optionally, a page also contains some tags (T), which are for the moment not fully implemented. Each element can be in one of four states: in need of content (C), in need of review (R), reviewed as bad (QB), reviewed as good (QG).
A flag is the status of a specific element. We always write the status first and the element in second. So the flag that denotes that some hint is in need of review is RH. This implies 12 flags (16 If we count the flags associated to tags).