All functions in the API-MS-Win-Core-UMS-L1-1-0 set are documented exports from KERNEL32:

All are new for version 6.1 and are exported from x64 builds only. The set is defined for both x86 and x64 builds in version 6.1 but only for x64 builds in later versions. All x86 distributions of Windows omit the API-MS-Win-Core-UMS-L1-1-0.dll file.

The Windows 7 schema redirects this API Set to KERNEL32.