I haven't heard that term in a while. That would be Allen Bradley PLC5/15 or PLC5/25 if my memory is correct.
I have not looked at this isue for at least 10 years, but I will explain what I remember.
This is mainly to do with the configuration of the Remote IO port.
Scanner Mode: The idea is you could have an I/O Scanner, which is a stand alone processor, and accesses or "scans" all the IO connected to it on remote IO, or in the local rack. This is the normal way to use a PLC.
Adapter Mode: I/O Adapter. In this mode, the processor can be connected on another PLCs remote IO network, and can be configured as an remote IO adaptor. Part of the memory or IO can be mapped to RIO addressing. To the scanner PLC, it is just seen as IO.
I hope this helps. For more detail on setting up the adapter mode, see the PLC5 processor manual.
Hope this is of some help.