Hi,
Dorico can use expression maps to decide which MIDI message will control dynamics in the VSL instruments.
There are up to two dynamic control messages for each entry of an expression map: a primary and a secondary one. According to the documentation and the discussion I have followed, this was mainly conceived for those libraries where you send an initial value via Velocity, then change it with Modulation (usually CC1).
Other cases are possible. The expression maps developed by VSL for the SYzdSE libraries include both the primary (Velocity) and secondary (CC2) dynamic control. I wonder if this is done so that you can load either the Velocity-controlled presets, or the VelXF ones, and Dorico can control the one or the other, due to both dynamic control streams are included in the expression maps.
Paolo