A subgroup is a sequence of one or more objects from the same group (Section 2.3) in ascending order by Object ID. Objects in a subgroup have a dependency and priority relationship consistent with sharing a stream and are sent on a single stream whenever possible. A Group is delivered using at least as many streams as there are Subgroups, typically with a one-to-one mapping between Subgroups and streams.
When a Track's forwarding preference (see Section 10.2.1) is "Datagram", Objects are not sent in Subgroups and the description in the remainder of this section does not apply.
Streams offer in-order reliable delivery and the ability to cancel sending and retransmission of data. Furthermore, many QUIC and WebTransport implementations offer the ability to control the relative scheduling priority of pending stream data.
Every object within a Group belongs to exactly one Subgroup.
When Objects are sent in a subscription (see Section 5.1), Objects from two subgroups MUST NOT be sent on the same stream, and Objects from the same Subgroup MUST NOT be sent on different streams, unless one of the streams was reset prematurely, or upstream conditions have forced objects from a Subgroup to be sent out of Object ID order.
Original publishers assign each Subgroup a Subgroup ID, and do so as they see fit. The scope of a Subgroup ID is a Group, so Subgroups from different Groups MAY share a Subgroup ID without implying any relationship between them. In general, publishers assign objects to subgroups in order to leverage the features of streams as described above.
In general, if Object B is dependent on Object A, then delivery of B can follow A, i.e. A and B can be usefully delivered over a single stream. If an Object is dependent on all previous Objects in a Subgroup, it likely fits best in that Subgroup. If an Object is not dependent on any of the Objects in a Subgroup, it likely belongs in a different Subgroup.
When assigning Objects to different Subgroups, the Original Publisher makes a reasonable tradeoff between having an optimal mapping of Object relationships in a Group and minimizing the number of streams used.