I've gotten the "Heavy" designation in my T-9, but I've only noticed it at surface ports (most recently, the robot ATC at Farseer). It was the three letter code that I've chosen, just with the addition of "Heavy." Not sure why it only happens at surface ports (in my experience), but it could have something to do with gravity (if you want to go with an in-game explanation). Starports can basically decide how "Heavy" a monster like the T-9 is, because they have artificial gravity. Surface ports, however, just have to deal with whatever the gravity is on that particular rock. That's the way I've been thinking about it, at least.
Back to the original question, other CMDRs are right. I chose "ship name" as my preferred callout, and it worked just fine (first three letters). I still haven't decided on a name for my Vulture, so the other day I got "Victor Uniform Lima." If you chose the "ship designation" as your callout, it will be whatever you've set it to in the Livery - the default looks like it is generated based on your CMDR name.