value can be an integer constant, a floating-point constant, or a floating-point variable. Using an integer variable will not cause a crash, but has the same effect as setVelLOF(0), which is not very useful.

Homeworld effects use cylindrical co-ordinates. LOF (Line Of Fire, as near as I can tell) corresponds to the Z axis. Think of a clock face. The Z-axis comes straight out of the clock towards you. setVelLOF() gives an object a speed of value meters per second along the Z axis.