SCTE 104/35 Codex

The unique_program_id in splice_insert identifies which specific instance of a program a splice belongs to, so ad scheduling can tie a spot to the program it was sold against rather than to a clock time. Time of day does not identify a program, because live schedules slip: sports, awards, and news run long, get moved, or get cancelled, and an advertiser who bought a premium program will not pay if the spot lands in ordinary programming instead. Tagging the event with a program id lets traffic systems match spots to the right program and substitute cleanly when the program is not where it was expected. SCTE 67 notes that new work should consider identifying programs with a time_signal plus a segmentation_descriptor and UPID instead, which gives a better-defined identifier.