diatonic = 7 notes on a scale, with 5 major and 2 minor intervals. Like the white "buttons" (or do you call them keys?) on the piano
There is in fact more different types of diatonic (different "modal" keys, where the major and minor intervals are in a different place). {It is different from f.e. pentatonic (like some dulcimers have)}.
You can 'only' play in one clear 'ordinary' key (on the push; the rest is to be found in the previous thread about push and pull).
Doric tone ladder is also on the pull, I guess. Ask a schooled mouth harmonica player and he/she will explain.
Chromatic is 12 scale (black and white "buttons" on the piano.) With this system you can play in all keys.
In the past it was called "well tempered". You didn't have to tune your instrument if you wanted to play in a different key.
So one row squeezeboxes are automatically "diatonic".
Two row boxes (exept the Irish C# D) have just two diatonic rows, but if you switch between the rows you can play more chromatic like. Three rows sometimes have "black" notes on the third row, while others have another 'diatonic'key.
Real Irish C# D can be played more or less chromatically, am I right Irish Bravehearts?
same thing. BC is only a half note difference. So theoretically you have a lot of 'black' keys or whatsever they call them; take the the piano as the example) on the B row. I checked it out on the piano. Key of B is played mainly (exept for the B and the E) on the black "keys" ( The only issue is (and therefore you should check it on the accordion) "are they on the pull or push"? This makes it playing the chromatic scale smootly or not.
But if you have a BC, you can give me the answer if I am right or wrong.
Both the C#/D and B/C configurations have all the notes of a 12-tone scale available, and can be played in many keys. Not all keys are easy, though, and the bass chords available would limit somebody who actually plays them (many players in the Irish style play basses very sparingly or not at all).
On multi-row in-out boxes: these are generally tuned "equal tempered" because it doesn't make sense to tune to a preferred key (as in "just" tuning or Cajun tuning) if you're going to take advantage of the multiple key possibilitys of the 3-row.
Therefore, even a LMMH four-reed triple row isn't going to sound quite Cajun if the tuning is equally tempered. Happy 4th!