Xybernetics JSR Program File Number
JSR (Jump to Subroutine) instructions direct the processor to go to a separate subroutine file within the ladder program, scan that subroutine file once, and return to the point of departure.

But what does U:5, U:6, U:7,U:8, etc. mean in a JSR instruction?
Xybernetics JSR Program File Number

File number U denotes Program File and the number succeeding “U:” is the File Number.

You can determine the File Number for each ladder by right-clicking any ladder and selecting the “Properties…” from the context menu.

Xybernetics JSR Program File Number

Which will pop up a window which will tell you what the File Number is for a particular Ladder.

Xybernetics JSR Program File Number