SCTE-35 (ANSI/SCTE 35 2013) is a joint ANSI/Society of Cable and Telecommunications Engineers standard that describes the inline insertion of cue tones in mpeg-ts streams. The full standard name is "Digital Program Insertion Cueing Message for Cable."
SCTE-35 is used in the US to signal a local ad insertion opportunity in the transport stream, and in Europe to insert local TV programs (e.g. local news transmissions).
Currently, SCTE-35 is erratically implemented. Different providers have different "interpretations" of the specification.