Hello Henry,
Please try to be a bit more specific when making questions, it is not clear at all what you mean or what you need to understand better.
That said, the manual page for seusers(5) contains a description of the structure of the file, basically:
user_id:seuser_id:range
In your case, you show two lines:
root:root:s0-s0:c0.c1023
and
__default__:user_u:s0-s0
The first field, the user_id, describes the mapping between Linux users and SELinux users. You can read more in semanage-login(8). The special designator __default__ applies to all Linux users for which there is not an explicit mapping defined.
In my Fedora:
$ sudo semanage login -l
Login Name SELinux User MLS/MCS Range Service
__default__ unconfined_u s0-s0:c0.c1023 *
root unconfined_u s0-s0:c0.c1023 *
The second field describes the mapping between SELinux users and SELinux roles. The manual page you can read is semanage-user(8).
In my system:
$ sudo semanage user -l
Labelling MLS/ MLS/
SELinux User Prefix MCS Level MCS Range SELinux Roles
guest_u user s0 s0 guest_r
root user s0 s0-s0:c0.c1023 staff_r sysadm_r system_r unconfined_r
staff_u user s0 s0-s0:c0.c1023 staff_r sysadm_r system_r unconfined_r
sysadm_u user s0 s0-s0:c0.c1023 sysadm_r
system_u user s0 s0-s0:c0.c1023 system_r unconfined_r
unconfined_u user s0 s0-s0:c0.c1023 system_r unconfined_r
user_u user s0 s0 user_r
xguest_u user s0 s0 xguest_r
It looks like you have modified the default SELinux user to be user_u.
Does that help?