I just stumbled over these two entries in the MQL4 Reference:
Returns true if Expert
Advisor runs in the Strategy Tester optimization mode, otherwise returns
Flag that a mql4-program operates in
non zero in profiling mode, otherwise
So is there a difference and if so what is PROFILE_MODE?
Of course, Profile mode as nothing to do with backtesting or optimization. It's a tool available in MetaEditor to profile your code, in other words to check the performance of your code.
There is no online information for MT4, but you can press F1 in MetaEditor (User Guide) and search for Profiler.