In Forex, why we can set trailing stop in Metatrader 4 without the stop loss? -

I am curious about this because i once used FXCM trading platform before and i must set SL first before setting the TS. But in MT4, i can set mt TS without SL. Anyone know the reasons? thank you! it is just the platform fascility. When ts is set by distance it will generate two things one stop loss and other trailing distance.