Shevuot 24B

(חשובי) [אחשובי]; אלא הכא מי אחשביה?!

in taking the oath, he granted significance to the non-kosher food, and that is why his eating forbidden items is considered to be eating. But here, where he took an oath not to eat and then ate non-kosher food, did he thereby give significance to it? For this reason, no conclusive proof can be cited from the last case in the Mishnah.

אמר רבא: מאי טעמא דמאן דאית ליה איסור כולל?

R' Yoḥanan understands the Mishnah as referring to a case in which the oath takes effect because it is a more inclusive prohibition than the prohibition against eating non-kosher food, in that it adds additional prohibitions for the same individual. In this context, Rava says: What is the reasoning of the one who holds that a more inclusive prohibition takes effect where there already is a prohibition in place?

מידי דהוה אאיסור מוסיף.

Rava answers his own question: It is just as it is in the case of an expanded prohibition, which incorporates additional people into the list of those for whom the original item is forbidden. An expanded prohibition also takes effect despite the fact that there is already a prohibition in place.

ומאן דפטר, דלית ליה? כי אמר איסור מוסיף ⁠– בחדא חתיכה, בשתי חתיכות לא אמרינן.

And with regard to the one who exempts another from liability in the case of a more inclusive prohibition, as he does not hold that it takes effect, why does he hold that an expanded prohibition does take effect? When he says that an expanded prohibition takes effect, that is with regard to a single piece, i.e., that the number of people who are prohibited from eating the same piece of meat is expanded. With regard to two distinct pieces, i.e., a case where the new prohibition expands the scope of prohibited items, as in an inclusive prohibition, we do not say that it takes effect.