This header file is specific for any particular target, and contains definitions and inline functions necessary to get the basics going. This header is included by <ucr/ucr.h> to get definitions for the thread_splx functions. (Applications should not include it directly.)
It is usual for this header to actually include for example <i960/ucr_asm.h>, or whatever is appropriate for your machine. There are several targets that have different configurations and so different target names and <ucr/config.h> files but the same CPU. The convention of calling the ucr_asm.h for the CPU reduces code duplication.