The debugger programmers interface is exported from from the "DEBUG-INTERNALS" or "DI" package. This is a CMU extension that allows debugging tools to be written without detailed knowledge of the compiler or run-time system.
Some of the interface routines take a code-location as an argument. As described in the section on code-locations, some code-locations are unknown. When a function calls for a basic-code-location, it takes either type, but when it specifically names the argument code-location, the routine will signal an error if you give it an unknown code-location.