Simple programming questions

When do you use the NPO line?

I'm a little confused... after doing a "callsub" and then the "RET", where does the program flow to? To "STARt", or the line after the previous "callsub" ?

You mean NOP line?
NOP is No Operation, doing nothing.
PLC program start from the line "Start" execute all the way to "End" then start from beging again. Over and Over never stops.

After executing TSTxy line, there are two braches the code can go, if it is false, the next line is escaped and jump directly to the line after next line. If your last line of PLC code is TSTxy, then you will need to add a NOP line, so that TSTxy can successful branch to the next instruction.