Variation Send is the amount of the current PART you want to send to the "Variation" effect.

There are 3 levels of effects - Insertion (there are two and are associated with each PART), System (there are two types: Reverb and Variation and are shared by all PARTs - with the option to send a different amount of each PART to the system-level effects), and Master (final stage of effects that all PARTs participate in without the option of changing the amount/level of participation by each PART).

https://www.yamahasynth.com/forum/21987-effects-application-of

This thread has a picture:

https://yamahasynth.com/forum/19195-fx-feedback-loop

Notice in the picture (left hand side) that the system effects SENDS are all per-PART. Each PART has its own sends.

Not as obvious from the picture is that the return and "Variation To Reverb" is at a common/system/global level. The Montage GUI does a better job of showing this. See the common routing screen:

Credit: Moessieurs.com

This is also where you edit what the Variation effect IS and what the Reverb effect IS. You can select one effect type for each. Variation was traditionally called "chorus" - but expanded to more than just chorus to include almost any effect (including many reverb types). Therefore, the effect was renamed "Variation". In the common/system/global routing area, you can assign what each system effect is and also here is where you set the return amount. The return amount is a MIX of all SENDs to each system effect (we can just focus on Variation). The SENDs (a little of one PART, a lot of some other PART, something between for a 3rd PART -- you set those at the PART level) will be mixed together. After mixing each SEND together - you can choose to RETURN a little - or a lot - or something between of the SEND MIX back to the signal chain (back to your resulting sound). There is only one RETURN for the entire system. This is sort of a "wet" level where you can adjust how much of the variation level overall you want fed back to the signal. Then there's also the "Variation to Reverb" option - where you can feed this Variation signal (the mix of all the SENDs) down to Reverb so the Reverb system effect applies reverb to a signal that has your variation effect applied (and set how much of this you want).

Thank you Jason for the credit 😉

To complete your detailled answer, here is the Moessieurs Generality page about FX :

http://www.moessieurs.com/fx-base-montage.html

It's in French, use the google translator tool in top banner, not perfect but enough good to understand basics 😉

Thankyou very much, Joel and Jason !