What are the meanings and implications of the various options for Status? (I would like to understand them all but I am particuarly confused/curious about the Status “Left”)
It’s not that easy to answer this question. 
Anyway, every Gibbon user has a current status. If e.g. a staff has status “Full” then he is currently working at the school. If the same person leaves the school the status should be changed to “Left”.
The status of a user is rather important and has a number of implications. It’s rather difficult to list all implications here. Using common sense will definitely help you here.
Generally the status determines how data of a user is displayed and made accessible in Gibbon, e.g. a student with status “Left” will no longer show in the fast finder or cannot be enrolled.