There are a couple controls that get put on a console that sometimes create confusion, as people don't fully understand how MIDI works on a Classic system. These are Multi MIDI and MIDI Registration Off.
Just as there are Combination Memory Levels in a system there are parallel MIDI Memory Levels (MML). Each MML contains all the parameters (patch, channel, MSB, LSB etc) for each MIDI Expander stop on the console. When you receive your Classic system all MML contain the same factory defaults, so if you change MML you won't see any difference. Incidentally, if you let us know in advance, we can pre-set the MIDI parameters to whatever you like. When you change your combo memory to get a different registration on your General and Divisional pistons, the MIDI Memory Level does not necessarily change too. Imagine two towers that are part of the same building, each floor in each tower is a memory level, each room on each floor contains the information about 1 particular thing. Unless you tell them otherwise, the elevators in the towers are going to function independently.
Multi MIDI is the feature that allows you to tie the elevators for the two memory towers together, such that if you change the Combo Memory level the MML will also change. You have two options with Multi MIDI, you can have a piston on the console somewhere (usually a control panel) so the organist can turn the function on and off quickly, or you can turn on a Soft Switch, such that when the console is turned on, the Multi MIDI function is automatically enabled. Which you choose will depend on how MIDI is going to be used on the console and if you have space for a piston.
If you want a few different MIDI configurations, but do not want these configurations to change every time you change the Combo Memory level, you can use MIDI Level Select (MLS) pistons. In other words, you want the elevators for the two towers to function independently, but you only want to use, say, the first 4 floors of the MML tower. Each MLS piston allows you to directly access a different MML without changing the Combo Memory.
MIDI Registration Off is another MIDI control that sometimes confuses people. MIDI Reg Off doesn't actually have anything to do with MIDI Memory Levels. MIDI Reg Off is used in conjunction with a MIDI Sequencer (Record/Playback). When you are recording something onto a MIDI Sequencer, either our built in sequencer or a stand-alone unit, any registration changes the organist makes will also be recorded. It doesn't matter if these registration changes are hand registered or via the combo action. When the MIDI Reg Off piston is ON, all these registration changes will be ignored on playback. This would tend to be used if the organist wants to try different registrations for a section of music, ie does this sound better with or without the mixture, or what if I add the trumpet?
So, when you are designing a new console think about how you want to access the different floors of the MLS tower, this will tell you what (if any) pistons you need. If you do not have any MIDI Expander stops on the console, you will never need to access any floors and the MIDI Reg Off is only ever used with a MIDI Sequencer.