STP means starting point. The value of STP is added to the offset of the oto, so it can be positive or negative. Preutterance and overlap are measured from the starting point, so if you manually change STP you need to adjust both of those accordingly.
UTAU will automatically change STP, preutterance, and overlap on very short notes, but these changes are invisible. If you want to see the exact numbers, press ACPT to stamp them on the note properties.