Page 78 small rule book. Fire points, if the vehicle moved at combat speed models count as having moved, and models may only snap fire if the vehicle moved at cruising speed.
So to put into a DE example:
if your venom is stationary you can fire the heavy profile for your trueborn's splinter cannon.
if you move your venom 6" you can fire your trueborn's Splinter cannon at its assult profile
if you move your venom 12" you can snap fire your trueborn's splinter cannon in either heavy or assault.
Archon of the Obsidian Kabal