SP (Seats Pending): Means that the Pax Terminal cannot determine a seat release because final mission/cargo details are unknown.
TBD (To BE Determined): Means that the Pax Terminal cannot determine a seat release because final mission/cargo details are unknown.
T (Tentative): Means mission/cargo details are planned but factors remain where the actual number of seats can not be finalized i.e. fuel weight, weather en route or even maintenance. The number could change (up or down). Some terminals also use "TSR" (Tentative Seats Released).
F (Firm): Means that all factors are known and the seat release is solid. However, due to changing mission purposes, flights and seats are always subject to change without notice.
For example: 0T means zero seats tentative, 10F means 10 seats firm etc.... don't count on these predictions as gospel as things can (and do) change at the last minute just prior to showtime and/or roll call.
Note: All the above numbers are after duty pax and cargo have been considered.