Archives of Nethys lists several siege engines with "light" in the name (light ballista, light bombard, light catapult, light trebuchet); I imagine it refers to those.
P.S. You have wandered into the subforums for the Pathfinder 2nd Edition Playtest. If you have followup questions I suggest you post in Rules Questions, as many more people read that regularly.
Thanks I moved it over. I thought about that just use ones with light in the title idea. But 1. That answer feels a bit lazy. 2. It feels inconsistent. I mean what magical difference is there between a heavy balista and a light one? It's still turn crank, load shot, aim and fire. And 3. I just find it hard to believe that a Gunslinger archtype built for siege warfare can't use a cannon.