Hi Cass

Check your manual!

As an example, imagine you have a Piano sound assigned to
Upper 1, and a Strings sound assigned to Upper 2. If you turn
the Expression switch on on Upper 2, and off on Upper 1, you
can use a continuous pedal to control only the Strings’ volume,
while the Piano remains unchanged.

STS are sound selections do you mean variations linked to sound selections. This can be changed using the STS mode button.

This button turns the Single Touch and Variation/STS Link
functions on or off.

On -- When a different Style (or the same again) is
selected, a Single Touch Setting (STS1) is automatically
selected. The Keyboard sounds and effects will change, along with the Style sounds and effects. Pad sounds will change too.

Flashing -- Variation/STS Link function activated. This function
makes each Variation recall the corresponding
STS when selected. For example, select
Variation 2, and STS 2 will be automatically
recalled; select Variation 3, and STS 3 will be
automatically recalled.

Off -- When you select a different Style (or the same
again), the Style sounds and effects will change,
as well as Pad sounds. The Keyboard sounds and
effects will not change.

Hope this helps

I have just acquired a PA800 which works in pretty much the same way. There is a renamed songbook I'll see if I can find it for you.