RUID, RGID - real UID and GID of the user on whose behalf the process is running

EUID, EGID - effective UID and GID used for privilege checks (except for the filesystem)

SUID, SGID - Saved UID and GID; used to support switching permissions ``on and off'' as discussed below. Not all Unix-like systems support this, but the vast majority do (including Linux and Solaris); if you want to check if a given system implements this option in the POSIX standard, you can use sysconf(2) to determine if _POSIX_SAVED_IDS is in effect.

supplemental groups - a list of groups (GIDs) in which this user has membership. In the original version 7 Unix, this didn't exist - processes were only a member of one group at a time, and a special command had to be executed to change that group. BSD added support for a list of groups in each process, which is more flexible, and this addition is now widely implemented (including by Linux and Solaris).

umask - a set of bits determining the default access control settings when a new filesystem object is created; see umask(2).

scheduling parameters - each process has a scheduling policy, and those with the default policy SCHED_OTHER have the additional parameters nice, priority, and counter. See sched_setscheduler(2) for more information.

limits - per-process resource limits (see below).

filesystem root - the process' idea of where the root filesystem ("/") begins;