Module "slot" locations in a Point I/O group are assigned in the order they are encountered from left to right. You may have a control system design that supports an option that is not always needed. This means there may be I/O modules that aren't needed and that could be left out of the system. But if you just remove them then the adapter will re-address the modules right of the opening. Enter the 1734-ARM. It is a module that simply holds a slot address for something else. The 1734-ARM and cheaper than any actual module, especially intelligent modules like analog and safety modules. So there is come cost savings to be had by using them.
Keith