MoQT maintains priorities between different schedulable objects. A schedulable object in MoQT is either:
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An object that belongs to a subgroup where that object would be the next object to be sent in that subgroup.
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An object that belongs to a track with delivery preference Datagram.
Since a single subgroup or datagram has a single publisher priority, it can be useful to conceptualize this process as scheduling subgroups or datagrams instead of individual objects on them.
A priority number is an unsigned integer with a value between 0 and 255. A lower priority number indicates higher priority; the highest priority is 0.
Subscriber Priority is a priority number associated with an individual subscription. It is specified in the SUBSCRIBE message, and can be updated via SUBSCRIBE_UPDATE message. The subscriber priority of an individual schedulable object is the subscriber priority of the subscription that caused that object to be sent. When subscriber priority is changed, a best effort SHOULD be made to apply the change to all objects that have not been sent, but it is implementation dependent what happens to objects that have already been received and possibly scheduled.
Publisher Priority is a priority number associated with an individual schedulable object. It is specified in the header of the respective subgroup or datagram, and is the same for all objects in a single subgroup.
Group Order is a property of an invidual subscription. It can be either 'Ascending' (groups with lower group ID are sent first), or 'Descending' (groups with higher group ID are sent first). The publisher communicates its group order in the SUBSCRIBE_OK message; the subscriber can override it in its SUBSCRIBE message. The group order of an existing subscription cannot be changed.