[SQ4 B5] Please clarify if "Default cost per RT" is being used in backtesting or if only "Commission" is used

Please clarify which values are used by the backtesting engine for the fixed costs per round-turn-lot. In the symbol settings, we can set "Default cost per RT" (is it in pips? Dollar?). But in the backtesting engine (for example in the Strategy Builder), we can set "Commission" in $. Is that on top of the "Default cost per RT" in the symbol editor or is the "Default cost per RT" not used at all? This is a bit confusing..... Why doesn´t the backtesting engine just use the value that is set in the "Default cost per RT" in the symbol editor? Why the extra "Commission" parameter at all?