I can't help with musical theory, but I've been in a number of guitar workshops where the term 'modal' has been used to indicate a tuning, or chordal arrangement, which avoids the third - i.e. doesn't sound overtly major or minor. I've no idea how this fits with real musical theory but it's a useful notion in practice. If our putative fiddle player above whispers to me in a session that a tune is 'modal' then I'll know that DADGAD tuning will be safe (if predictable!) most of the time. With a little help from the capo, of course...