You can write "sy" whereever you like, the mission won't end. There is a mistake in the official tute, so if you do exactly what it says the mission won't work. You need to write something else. I can't remember what but its in my tute.
To find the official tute click on
Start - Codemasters - OFP - Readme
and click on the Editor tab.
Edit: as I recall sy was a variable.