When an event has shifts, you may modify the displayed shift setting under "Settings" when you "Edit" an event, so displayed shifts are either Open or Future. You can also change how shifts are ordered (chronologically or alphabetically).
Open = A user can still register to that shift
Future = The shift is in the future. Might be full, might not. Registration might be closed, might not.
As a individual looking at the event with shifts, they may override the above default and filter by the following:
All = Show all shifts - by default what an event uses
Open = As shared above.
Full = Shifts are full, Future or Past
Future = As shared above.
Closed = The shift could be in the future, but registration has closed for it
Over = Shifts in the past