Terminology

GUID (Globally Unique Identifier): A 128-bit identifier with global uniqueness guarantees. Used to identify registry hives, layers, object types, and other entities that require stable identity across systems and reboots.

LUID (Locally Unique Identifier): A 64-bit identifier with boot-scoped local uniqueness. Used to identify transient entities such as logon sessions and privilege instances that do not persist across reboots.

Nil GUID: The GUID with all 128 bits set to zero. A sentinel value meaning "no GUID" or "unset."

Nil LUID: The LUID with all 64 bits set to zero. A sentinel value meaning "no LUID" or "unset."