Function fetch_has_subgroup_id 

fn fetch_has_subgroup_id(flags: u64) -> bool

Whether an explicit Subgroup ID field follows the Group ID.

The DATAGRAM bit wins over the mode bits. Section 10.4.4.1: “When 0x40 is set, it SHOULD set the two least significant bits to zero and the subscriber MUST ignore the bits.” A publisher that sets the bit and leaves the mode at 0x03 anyway has written no Subgroup ID field, so reading one would consume the Object ID in its place.

Both end-of-range values carry mode 0x00, which is also what Section 10.4.4.2 requires of them, so they need no case of their own here.