The MIDI standard uses Continuous Controller 7 for volume. Few plug-ins adhere to the MIDI standard completely but CC7 is pretty common. Read about how to work with CC data
here. The link is for the X3 help pages. That is the most recent version I found you mentioned using in a previous thread. Please make sure when posting to specify which DAW and version the question is about.
Volume may be automated on the audio track too.