> Why does NFS care about the order of groups?

Because it has to select a limited subset to send over the wire, and
historically it has simply picked the first ones from the list.

This is actually mostly moot at the moment, because NGROUPS_MAX is 16,
and the groups count limit imposed by the wire protocol is...16.
However, it's not entirely moot, because an NFS mount can specify a
lower limit for the max group count sent over the wire (see
NFSMNT_MAXGRPS).

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