The editor works in the D-20's Multi-Timbral mode. Performance mode works a little differently, and doesn't exactly apply to what I outline here.
First off, the difference between a Tone and a Timbre: A Tone is where the real meat of a sound is set up. In the patch editors of Patch Base for the D-20, you're editing Tones (selecting the sample, changing the Filter, Amp, etc). A Timbre is just a little bit of info that selects which saved Tone (from the synth's memory) should be played, changes the tuning if you want, and turns Reverb on or off.
The D-20 has 3 banks of Tones: A, B, and Internal (or "Int"). A and B are just read-only presets. The Internal bank is the only one you can write to. So, when you go to the "Tone Bank" tab, you're fetching or sending the 64 tones in the Internal Tone Bank, since A and B can't be changed.
The D-20 also has 128 Timbres that can be stored. When you select a Timbre on the synth, those 128 are broken up into A and B banks. So the first major point of confusion (for me at first, and others) is that there's an A and B bank of Timbres, and also Tones. But for Timbres, that A and B bank are the ones you can save to, whereas for Tones, A and B are the presets.
On the synth, when you select A or B and hit a number, you're recalling a saved Timbre, which in turn loads a Tone. What you're doing is essentially updating the Timbre data for your selected Timbre (which you can also see in the Timbres tab). E.g., try selecting a Timbre on the synth itself, then doing a "Fetch" in the Timbres tab, and you'll see that the Timbre data changes in the editor.
Patch Base doesn't support saving Timbres yet. It needs a "Timbre Bank" tab added that will allow for that. This is something I plan on adding, and I'm sorry it's not there yet. These synths are just a very different structure from pretty much all others, and I've had a hard time designing around it.
In Multi-Timbral mode, you have 8 different loaded "Timbres" you can play. Timbre 1 is played on MIDI Channel 1, Timbre 2 on channel 2, etc. This is hard-wired in the D-20. Each Timbre has 1 Tone assigned to it.
So, in the Timbres tab, all you're really doing is specifying, for each Timbre, which saved Tone should be used. Tone Group picks which bank you're choosing from (presets vs. internal), and Tone Number is the number of the tone used. In an update I'll make it so that "Tone Number" is actually showing you the names of the Tones instead of just the number.
The Tone 1, 2, 3, etc tabs are there to fetch and edit the currently loaded Tone for each of the 8 Timbres.
So the best way to use the editor right now is to design sounds in the Tone 1, 2, etc tabs, and when you have some you want to save, save them to the Internal bank on the synth. Then, set up your Timbres to use those sounds that you want, so that when you load a Timbre by selecting A or B and hitting the number buttons, you're loading a Timbre that loads the Tone you created. BUT, saving that Timbre configuration is still something you have to do on the synth itself. You can edit the Timbres from Patch Base, but the saving you have to do manually.