The Scheduled Time of Arrival (STA) tags, which are still being rescheduled by the Time Based Flow Management (TBFM) calculations. The aircraft will remain unfrozen until the time the corresponding Estimated Time of Arrival (ETA) tag passes the preset freeze horizon for that aircraft's stream class. At this point the automatic rescheduling will stop, and the STA becomes "frozen."

Source: Pilot Contoller Glossary (PCG)