Yamaha TG500 Owner's Manual page 219

1a, 1b, 2a, and 2b (Send switches)
Determines to which of the EFFECT 1 and EFFECT 2 effect stages the
output from the current layer is sent. The [-1] and [+1] keys can then be used
to turn the selected stage on or off.
If a "single" type effect is selected then only stage "a" can be selected. If a
"dual" or "cascade" type effect is selected, then both stages "a" and "b" can be
selected. An effect stage that cannot be selected is represented by "--" on the
display.
See the "EFFECTS" section beginning on page 251 for more details.
MULTI EDIT MODE / 4: Effect
4-03: SEND 207