Hmm, I'm not quite sure what those terms mean, but a quick search seems to suggest to me that it is a "Dental" (not alveolar) vs retroflex.

See 00:53 of this video: https://www.youtube.com/watch?v=rXFx3Ly_imY

In that table, त is the first column of the Dental row and थ in the second column of that same row (aspirated?). Similarly, ट is the first column of the Retroflex row and ठ is the second column of that row.