Hi!
Im currently fascinated by the IBM 5161 expansion unit.
According to the technical manual, the extender card needed to connect the unit maps some new functions in the I/O space.
There are things like:
Latching an address in the expansion unit
Reading the adress on the ISA bus in the expansion unit
Latching data on the bus of the expansion unit
Reading data of the bus of the expansion unit
Disable/enable the expansion unit
Check the status of the expansion unit

These can be done by reading/writing I/O address 0x210 - 0x213 or writing to the memory area 0xFXXXX.
For more info on this you can read the documentation of the device.

My question is now:
Have these features ever been used? Are they just some ideas that have been proven useless and take up I/O adresses and memory area?

They are not required at all for the PC Bus expansion, besides perhaps the enable unit function that isnt really necessary either.

It just feels as such a strange thing to do and it just uses memory and IO areas without any use case.
Best regards,
Niko