HSO was originally a standalone VSTi, and the documentation for that version was much better. Below is an extract from that manual that might help explain why this apparently strange control method was adopted:
"_If HALion Symphonic Orchestra is your first professional orchestra li-
brary, if you are not used to conducting, and if you’ve always used or-
chestral sounds from hardware samplers or keyboard workstations,
you will have to forget about a couple of “bad” habits – using velocity
as your main controller for controlling expression and dynamics for ex-
ample. While velocity is appropriate for controlling dynamics for drums,
keyboards or synthesizers, orchestra sections and especially string in-
struments are a totally different story.
When playing drums, pianos, guitars or other percussive instruments,
the volume and intensity is defined by the attack, after that there are
few options to change anything related to dynamics.
With bowed instruments, this is totally different. Strings need expres-
sion control all the time, and most of it is happening after the initial at-
We have introduced a special Crescendo controller allowing you to
continuously control the expression while you are working. It not only
controls the volume, but also the choice of the layer played according
to the controller value (i.e. the actual pianissimo, piano, … forte, fortis-
From now on bear in mind: When playing and arranging with HALion
Symphonic Orchestra, for many instruments the crescendo controller is
much more important than velocity_."