Prior Group ID Gap (Extension Header Type 0x40) is a variable length integer containing the number of Groups prior to the current Group that do not and will never exist. For example, if the Original Publisher published an Object in Group 7 and knows it will never publish any Objects in Group 8 or Group 9, it can include Prior Group ID Gap = 2 in any number of Objects in Group 10, as it sees fit. A track with a Group that contains more than one Object with different values for Prior Group ID Gap or has a Prior Group ID Gap larger than the Group ID is considered malformed. If an endpoint receives an Object with a Group ID within a previously communicated gap it also treats the track as malformed (see Section 2.5.
This extension is optional, as publishers might not know the prior gap gize, or there may not be a gap. If Prior Group ID Gap is not present, the receiver cannot infer any information about the existence of prior groups (see Section 2.3.1).
This extension can be added by the Original Publisher, but MUST NOT be added by relays. This extension MUST NOT be modified or removed.