I certainly can....
When a ControlLogix controller uses I/O modules in a remote chassis, without Rack Optimisation, it will use a "connection" for each and every module.
The system only allows 250 connections, and some of these will be used by comms and other modules. In a large system it would be easy to run out of connections.
Rack Optimisation reduces the connections needed for digital modules only, by getting the remote I/O communications module (ControlNet or EtherNet Bridge module), to act as a Remote I/O adaptor, gathering digital inputs from the chassis, and delivering digital outputs.
This is done via two tags, automatically created when you configure the remote comms module for Rack Optimisation.
The whole of the digital I/O for a chassis is then exchanged with the controller using just 1 "connection".
In contrast, if you had a 17-slot remote I/O chassis with a mix of digtal I/O in all the slots (barring the comms module of course). Without Rack Optimisation, you would consume 16 connections to exchange the I/O data.
The tag-names for the I/O in the remote chassis change, and don't look like proper I/O tags, but the system caters for this by automatically creating alias tags to the rack-optimised tags, allowing the programmer to continue to use standard Location : Slot : Data I/O tag addressing.
An example is called for.....
Bit 0 of an Input Module in slot 3 of a remote chassis, connected via a CNB module in that chassis with a name of Remote, would be addressed like ....
Remote:3:I.Data.0
With Rack Optimisation, the data is brought into the controller from the CNB in another tag, called the rack-optimised tag...
Remote:I.Slot[3].0
The system creates an alias from the I/O tag to the rack optimised tag for you, so you can address the input point like all others.
Hope This Helps...