Phase
Phase (not to be confused with QuestPhase - which is 8-bit and denotes Quest progress) is a large unsigned 32-bit integer (a number between 0 and 4,294,967,295) which combined with flags and watch rank serves to tell the game how far you are into the story.
The current phase can be obtained in CExpressions via the GetPhase() function.
Phase is actually two numbers, the chapter number and the subphase; when GetPhase is called it returns (ChapterNo * 1000) + SubPhase for simplicity. The following sub-pages will, for each game, list some common phase boundaries and what part of the story they belong to, for reference.