The file extension
<os>
used here indicates the operating system that is used, i.e.
texuser.<os>
becomes
texuser.dos
for MS-Dos,
texuser.w31
for W
I
N
D
O
W
S
3.1,
texuser.w95
for W
I
N
D
O
W
S
95, and
texuser.nt
for W
I
N
D
O
W
S
NT