What does 'full polyphony' mean?

Full polyphony means that you can play ALL of the notes within the range of the instrument at the same time. For example, an acoustic piano has 88 keys and it has, of course, the full polyphony, so you can play all of its 88 keys and hear them play all at the same time.

On the other hand, digital instruments, especially those based on sample playback, usually have a limited polyphony and they have a way to assign the available voices to the playing keys that is different from the 'one key = one note' concept.

Let's keep the acoustic piano as our example, if you press the sustain pedal and hit a note, you are playing one of the 88 available voices, and if you hit that note again, the stings keep vibrating but you're still playing THAT voice. With sample-based instruments it's not working like that, since you employ a new voice every time you hit a note, that's why some digital pianos offer 128 or even more voices even though they only have 88 keys.

With physical modeling syntesis the polyphony concept is very similar to the acoustic world, for example with instruments like GSi's MrRay, MrTramp and EG70 the available polyphony is exactly the number of playable notes, 73 for MrRay and EG70 and 64 for MrTramp.

And what about VB3? Just like the original Hammond organs, VB3 lets you play all 61 notes of the upper manual, plus all 61 notes of the lower manual plus all 25 bass pedals, total 147 notes. You don't need 256 or 512 voices for the polyphony since the instrument is not sample-based and doesn't re-allocate the voices like a digital piano does. One key = one note.