Hi Tom, Welcome to Yamaha Synth!

The MASTER KEYBOARD function would allow you to prevent the current (single) Performance from sending (transmitting) Bank Select and/or Program Changes when you recall that single Performance.

If you would like to turn the Program Change transmit function OFF globally, we recommend you not use the MASTER KEYBOARD function, but rather approach it this way:

Press [UTILITY]
Use the UP/DOWN CURSOR to select 02: MIDI
Press [ENTER] to view your option screens
Use the UP Cursor arrow to go to the *top* screen in the MIDI area.
*(you can tell it's the top screen by the arrow on the far right side: when only a 'down' arrow appears you are at the top of the list, if both an up/down arrow appears you have screens above and below, and you reach the bottom of a list when only an up arrow appears.)

You don't tell us exactly what you have going but you can set the "TX/RX/SW = OFF" (use the Right Cursor arrow to select this parameter, then turn the Data Dial or use the [-1/No]/[+1/Yes] buttons to make the parameter selection.

We assume you want to play external devices without sending Bank and Program Changes...this will accomplish that goal.

Extra Credit
If you wish to setup your own custom Performances and independently determine which region of the keyboard you are transmitting OUT via MIDI then the route to go would be to program each Performance's MASTER KEYBOARD setup as you require. Here's why I mention that, the Method where you are turning the global Bank Select and Program Change OFF means your CP4 Stage will use the "global" MIDI transmit settings which fix the MAIN PART to Ch 1, the LAYER PART to Ch 2, and the SPLIT PART to Ch 3.

You may discover that a Split Performance is transmitting Ch3 from the left hand and Ch1 from your right hand when the global Utility settings are in play. When you select a Layer Performance you are transmitting simultaneously on Ch1 and Ch2 across the entire key range. So you would use the Master Keyboard function found within each Performance to customize your control over your external devices according to how you want to control them.

When you activate the Master Keyboard function, you are programming the singular (current) Performance location. Once set to ON, the Master Keyboard settings you make per Zone override the global (Utility mode) settings for Midi. But just for *this* Performance location. Remember each of the 128 Pergormance locations is individually customize-able.

Either you program the Master Keyboard setup for the Performance or it accepts (inherits) the current global settings.

If you still have questions about the behavior, post back here. But that should get you started.