Subroutine, Call program, If statements

I am setting up some simple automation to feed parts into a CNC router.  Once parts are fixtured I want to use an input signal (coming from an external PLC) in an if statement to call the right program for that fixture.  All the examples I see in the manual for IF statements us a parameter to compare to some value. Are parameters the only options you can use in an IF statement?  If so, I would assume I need to map an input signal to a system parameter. If I set a input signal to a digital input does that automatically create a system parameter I can use? Is there a more in depth manual to go over conditional statements somewhere?  

Assuming you use digital IO between lcnc and your PLC, you can configure say 4 signals in a BCD pattern based on which fixture is loaded. This can give you 16 fixture options.

In lcnc you can use mux16 component to convert the BCD to a float value which you can read in your NGC program using m67 and compare in an if statement

Edit.....sorry. looks like you can only output analog values from NGC program....ignore the above....

All I need to do is read an input (not that the card matters in this discussion but I will be using a 7i76E) and make a program selection. So if the mux16 works to read an input then we should be good. I am not outputting anything to the PLC, if I am, it would be standard M code stuff, but that pretty straight forward.

I think so. Looks like M66 should do what I want. Essentially I have a PLC controlling a infeed conveyor/fixture for some wood sheet parts. Once the part is fixtured the PLC will output a ready signal. There are multiple conveyor lanes incoming to the CNC, so I just want the main code to loop/scan for the ready signal to machine that fixtures part. The signal itself is a Digital signal, just on or off.