Yoma 51A

בכור ומעשר, דחלין על בעל מום קבוע, ואין יוצאין לחולין ליגזז וליעבד. אלא שם זבח לא קתני.

e.g., a firstborn or an animal tithe, the sanctity of which takes effect even on a permanently blemished animal, and this offering cannot vacate its sanctified status and assume non-sacred status for its wool to be sheared and to be worked. Rather, you must say that the baraita is not teaching a general category of sacrifices, but when it states: Offering, it is referring to a particular one.

ומאי שנא? תמורה ⁠— שם תמורה אחת היא. זבח ⁠— איכא בכור ואיכא מעשר.

The Talmud asks: And what is different about the two statements, i.e., why does the tanna deal with a specific case in one area, but a general category in the other? The Talmud explains: Substitution is one category, as there is no difference between one case of substitution and another. By contrast, with regard to sacrifices, there is a firstborn and there is the animal tithe, whose halakhot differ from other offerings, and therefore one cannot establish a single general principle. Consequently, the tanna certainly is referring to a specific offering.

ולרב ששת, אדמוקים לה באילו של אהרן, לוקמה בפסח, דדוחה את השבת ואת הטומאה ועושה תמורה, דקרבן יחיד הוא? קסבר: אין שוחטין הפסח על היחיד.

The Talmud continues the previous discussion: And according to the opinion of Rav Sheshet, who explains that the offering in question is not the bull of the High Priest but his ram, rather than establishing and interpreting this baraita as referring to the ram of Aaron, let him establish that it deals with the Paschal offering, which overrides Shabbat and ritual impurity and one can perform substitution for it, as according to all opinions, it is the offering of an individual. The Talmud answers: Rav Sheshet maintains that one may not slaughter the Paschal lamb on behalf of an individual, but only for a group. This means that it is not an offering of an individual but, at the very least, that of partners. For this reason, one cannot perform substitution for a Paschal lamb.