What does the 'X' flag signify in smbpasswd?

I have recently added a user to 2 servers running samba. When I compared
the smbpasswd files on the two machines, there is an extra flag, X, in
the flags field on one of the machines.

man 5 smbpasswd only lists U, N, D and W as flags.

There are no problems with the user accessing the shares on either server
from Windows, but I am curious as to what the flag signifies.

--
Rinso
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