As you know I dont get to do this as much as most PLUS I have to deal with many brands. I more or learned the most of what I know from dealing with AB and the PLC5.
The plc5 offered options on slot addressing...half slot, 1 slot or 2 slot addressing. Each of these basically state what the bit addresses will be dependent on the slot it is in. An explanation of this can be found in this manual:
http://www.ab.com/manuals/cp/1785621.pdf
I ran into situations many times where other plc's would allow the address to be whatever the programmer desired, I didnt realize GE Fanuc 90-30 did this until Steve Bailey pointed it out to me recently.
Its not that you have to setup an AB unit to obtain this but like most rack style plc's you setup the hardware (many newer models will auto configure) and the addressing is automatically applied to the card type in the slot, depending on type...a 2 slot addressing model if I am not mistaken (been awhile) can have an input card in slot 1 and output in slot 2 but have same kind of address..ie I:1/1 and O:1/1. I am sure Roger or some of those that deal with this on a more regular basis will clear this up.
Overall tho if you see an input card in slot 0 then you know the addresses in that card will be I:0/x etc or written similar, the way its written can be different dependent on model.
All this makes more sense to me than someone that develops a system in a rack model with a space between modules then programs...later comes back and adds a card (output in this case) to a slot before the others but the addresses are higher in sequence.
Its also possible in RSLogix for the addresses to be automatically inserted in sequence if you want that, I dont use that feature because I have a tendency to jump around when programming and "attempt" to keep addresses associated with rungs/functions.
Alas my last attempt with GE Fanuc is an eyesore, its a small program and I understand it plus have it documented where I think it would be easy to troubleshoot but its still a mess.