The sliders have "fixed" functions. That's plural - so the sliders can be in one of a finite number of modes.

One of those modes is to control channel volume meaning MIDI channel 1-8 (if you have parts 1-8 selected vs 9-16). Internal or not - it will control MIDI channel 1-8 or MIDI channel 9-16 when in channel volume mode. So anything on the same MIDI bus will respond to the MIDI channel volume being set assuming you do not have MIDI volume filtered out of Montage's transmit.

You can set the sliders to adjust element volume - there's a mode for that. Since elements is an internal and not external paradigm - this should keep sliders from affecting external equipment.

Also, you can stop transmitting volume which is one way to keep it separated.

See: https://yamahasynth.com/forum/how-to-assign-various-cc-s-to-the-faders-and-knobs-on-the-montage-and-save-it-as-a-preset

for some answers.

See:

https://www.yamahasynth.com/forum/assigning-element-volume-to-sliders

for having sliders affect element volumes

There is some info here:

https://www.yamahasynth.com/forum/montage-as-control-surface-for-vsti-s/latest

Namely (from BM):

The Faders on the Montage send cc007 when "Part 1-8", or "Part 9-16" is lit. When "Element/Operator" is lit they send System Exclusive parameter change messages.

... which is why one way I say to get Montage sliders "out of the way" of controlling external volumes is to put them into element mode (assuming AWM2 here, operator mode = same mode - but in context of FM-X).

Got it, will give this a try. thanks for the response.