GUID used to "CustomMode" variable for two Secure Boot modes feature: "Custom" and "Standard".
Standard Secure Boot mode is the default mode as UEFI Spec's description.
Custom Secure Boot mode allows for more flexibility as specified in the following:
Can enroll or delete PK without existing PK's private key.
Can enroll or delete KEK without existing PK's private key.
Can enroll or delete signature from DB/DBX without KEK's private key.
Include/Guid/AuthenticatedVariableFormat.h

GUID Information

GUID's Guid Name	GUID's Guid
gEfiCustomModeEnableGuid	{ 0xc076ec0c, 0x7028, 0x4399, { 0xa0, 0x72, 0x71, 0xee, 0x5c, 0x44, 0x8b, 0x9f } }

Refer to Header File

SecurityPkg/Include/Guid/AuthenticatedVariableFormat.h