What is the ruling on making up for Ramadan for one who broke their fast for an illegitimate excuse, and what is meant by the "restricted obligation"?

Fasting missed Ramadan days is obligatory. However, jurists have differed regarding the specific timing for making them up:

The Hanafis hold that making up Ramadan fasts is an absolute obligation, and it is permissible to delay it as long as there is no strong presumption that one will be unable to make it up. There is no sin for delaying, nor is there any expiation (fidyah).

Whereas, non-Hanafis believe that making up the fasts is obligatory between two Ramadans. If one delays it without a valid excuse, then an expiation for the delay becomes binding, which is feeding a poor person for each day.

Some jurists differentiated between one who missed fasting due to a legitimate شرعي excuse, for whom making it up is on a relaxed basis (على التراخي), and one who missed it without an excuse, for whom there are two views: either making it up is on a relaxed basis, or it is immediately (على الفور), and this latter view is the correct one (الصواب).