Possible Clues:

• Unfilled slot abbr
• Up in the air, on a schedule: Abbr
• Not yet scheduled (Abbr.)
• Not yet decided, on a sched.
• Up in the air, for short
• Unfilled, on a sched
• Not yet known, for short
• As yet unscheduled: Abbr.
• Schedule-slot abbr.
• TV schedule notation