The LET option allows marking the output module executable even though certain error conditions were found during binder processing. When LET is specified, any exclusive reference (valid or invalid) is accepted. At execution time, a valid exclusive reference is executed correctly; an invalid exclusive reference usually causes unpredictable results.
Also with the LET option, unresolved external references do not prevent the module from being marked executable. This could be helpful when part of a large program is ready for testing; the segments to be tested might contain references to segments not yet coded. If LET is specified, the program can be executed to test those parts that are finished (as long as the references to the absent segments are not executed). If the LET option is not specified, these unresolved references cause the module to be marked not executable.