Active Directory support a number of different formats for the Base DN field, these are Distinguished Name, GUID and SID 

Distinguished Name is based on RFC 4514 e.g. CN=user1,CN=users,DC=domain,DC=com

GUID provides the GUID of an object which will be used as the base for searches, i.e <GUID=01f04883-d68e-4367-8ad1-a2faa79a2e5a>

SID format is the same as GUID but the entry is based on a SID i.e. <SID=S-1-5-21-2816452191-2840564649-4223122534-1000>

The SID and GUID option with the Search Scope set to Base Level can be used as quick search for users or other objects in the AD based on the GUID or SID.  The SIDs and GUIDs are entered in standard readable format.