Ksp reference Manual

Instrument
level, enter 
-1
.

The slot index (zero based) of the specified parameter. It applies only to group/
instrument effects, modulators and modulation intensities.
For group/instrument effects, this parameter specifies the slot in which the effect
resides (zero-based).
For modulators and modulation intensities, this parameters specifies the index
which you can retrieve by using:
find_mod(,)
For all other applications, set this parameter to 
-1
.
Engine Parameter Commands
125
KSP Reference Manual

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this parameter applies to instrument effects and to internal modulators.
For instrument effects, this parameter distinguishes between
1: Insert Effect
0: Send Effect
For busses, this parameter specifies the actual bus:
$NI_BUS_OFFSET + [0-15] one of the 16 busses
For internal modulators, this parameter specifies the modulation slider which
you can retrieve by using:
find_target(,,)
For all other applications, set this parameter to