Notes & justification
On the rāʾ of these three words: joined, it carries its kasrah and all read it thin (tarqīq); stopped upon, the kasrah is gone and giving the rāʾ its full body (tafkhīm) is, in the couplet's words, the more universal and inclusive position.
Thinned it is, bearing a kasrah, when they join; and giving it fullness at the stop is the more comprehensive, more inclusive way.