A module’s parameter values can be manipulated by mechanisms other than
direct keyboard or mouse input. These external adjustments are called
modulation. The mechanisms providing modulation are called
modulation sources, and the parameters that
they modulate are called modulation targets. Modulation sources can be
assigned to modulation targets by a number of mechanisms, but modulation
assignments appear in the Mod Matrix,
where they can be viewed and edited. Modulation assignments can be made by
right-clicking a parameter and selecting Edit Modulation..., by clicking
and dragging a modulation source onto a target, or by manually selecting
the sources and targets from the Selectors within the Mod Matrix itself.
A pad holds two parameters, so right-clicking it offers one Modulate
item per axis, such as Modulate Scale X... and Modulate Scale Y....
To drag a source onto a pad, drop it on the X or Y value beside or below
the pad, not on the pad itself.
Related: Modulation Sources, MIDI and OpenSoundControl Learn.