S-1-5-21-do-ma-in-503

S-1-5-21-…-…-…-503 is the (well known) SID for the Default Account (aka Default System Managed Account or DSMA).

The Default Account is needed to run multi-user-manifested-apps (MUMA apps).

The Default Account is member of the System Managed Account Group whose SID is S-1-5-32-581.