In case you use perforce View Mappings to obtain several disjoint parts of your project as in:

//repo/project/engine/... //jenkins${NODE_NAME}${JOB_NAME}${EXECUTOR_NUMBER}/engine/...

//repo/project/libx/... //jenkins${NODE_NAME}${JOB_NAME}${EXECUTOR_NUMBER}/libx/...

//repo/project/engine_test_files/... //jenkins${NODE_NAME}${JOB_NAME}${EXECUTOR_NUMBER}/engine_test_files/... 

The option "DELETE generated files" does nothing (the files are not deleted).

It might be useful to note that in such a case the p4 clean command also does not work when ran in the workspace root for example /var/jenkins/workspace/EngineTest. However it will succeed if ran independently in /var/jenkins/workspace/EngineTest/engine, /var/jenkins/workspace/EngineTest/libx, and /var/jenkins/workspace/EngineTest/engine_test_files, nevertheless still leaving the root workspace folder not cleaned.**