Slot numbering for Allen Bradley PLC are fundamentally important to identify where your cards are on the back-panel.
Here are picture of PLC that identifies how the slot numbers are allocated.
Allen Bradley-Rockwell ControlLogix
Starts with slot zero (which is normally CPU) and so on. Power supply does not have slot number.
The picture below shows with CPU in slot zero and EtherNet/IP card in slot 1
Allen Bradley-Rockwell PLC-5
Same as ConroLogix, starts with slot zero (which is normally CPU) and so on. But for PLC-5 it is identified as C0, C1, C2 and so on.
The picture below shows with CPU in slot zero (C0) and power supply in slot 3 (C3)
Allen Bradley-Rockwell SLC
The first slot is for CPU (as indicated in the picture below) and the rest if for I/O card.