Minecraft wiki entry for Universally_unique_identifier, associated text: 0xx is the "variant 0" of Apollo NCS 1.5, an old type of UUID.
10x is the "variant 1" of IETF RFC 4122 (Leach-Salz). It's referred to as "variant 2" by Java.
110 is the "variant 2" of old Microsoft products. It's referred to as "variant 6" by Java.
111 is the "reserved variant". It's referred to as "variant 7" by Java.