Vacuum backend with backend_xmin?

Hi,

This is a VACUUM FREEZE process.

-[ RECORD 1 ]------+--------------
pid | 129471
datid | 16401
datname | feed
relid | 1889166
phase | scanning heap
heap_blks_total | 1254901
heap_blks_scanned | 1017524
heap_blks_vacuumed | 0
index_vacuum_count | 0
max_dead_tuples | 11184809
num_dead_tuples | 0
backend_xid | <NULL>
backend_xmin | 3267908740
age | 8572

The query is:

select v.*, a.backend_xid, a.backend_xmin, age(a.backend_xmin)
from pg_stat_progress_vacuum as v join pg_stat_activity as a on a.pid=v.pid

Now, my question is why does a vacuum backend have a backend_xmin? I am
just curious.

Thanks,
Torsten