Heuristic level (HL) refers to the way knowledge is actually stored, and implemented, in Cyc. This stands in contrast to the epistemological level (EL), which refers to the way knowledge is expressed when Cyc communicates with users or external programs.
The use of HL versus EL sometimes seems to translate to “solve a problem by programming” versus “solve a problem by OEing”.
Another name for this might be “implementation level”.