Syntax to read an output variable and how to use with mobiflightsimvar_led

[JLD95]

Hi,
I don't understand how to use a output variable.

On https://hubhop.mobiflight.com/presets/, I see this preset PMDG B737 Autopilot A/T Arm Light
with code (L:switch_3801_73X, number) 0 >
and with description "PMDG B737 Autopilot A/T Arm Light 0 = off, 1 = on (armed)"

Shall I declare a new variable in events.user.txt in order to use for a "mobiflightsimvar_led" command ?

and what is the syntax for declaring an output in events.user.txt ?