Yamaha MU80 Owner's Manual page 52

MULTI MODE
Master Volume (M.Volum)
Master Attenuator (M.Attn)
Reverb Return (RevRtn)
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example, if you are using more than one MU80, set a different De-
vice Number for each. This is especially important when using the
data dump features. (See page 102.) If you have only one MU80, set
this to "all."
Graphically indicates current Master Volume setting.
This determines the overall Volume of the Parts.
Graphically indicates current Master Attenuator setting.
(maximum volume)
This determines the level of all Parts, but functions as an attenuator; the
greater the value, the lower the volume. This is useful when you are
playing several songs and want to keep their overall level consistent.
Graphically indicates current Reverb Return setting.
This determines the amount of Reverb return in the overall mix.
(minimum volume)