Numbers 19:3

Elozor. The mitzvah was [performed] by the assistant [kohein gadol].

Outside the camp. Outside the three camps.

[He] shall slaughter it in his presence. The commoner slaughters while Elozor observes.

And thou shalt give her unto Elazar, the chief of the priests, who shall lead her alone without the camp, and set round about her a railing (border) of the branches of fig trees; and another priest shall slay her with the two signs before him, after the manner of other animals, and examine her by the eighteen kinds of divisions.

AND YE SHALL GIVE HER UNTO ELEAZAR THE PRIEST. “[The proper fulfillment of] the commandment is [only when it is done] by the deputy High Priest.” This is Rashi’s language. But his intention is not to say that the obligation to perform this commandment [always] devolves upon the deputy High Priest rather than devolving upon the High Priest or any ordinary priest; but that this was a [special] temporary command in the case of the first Red Heifer that it should be done by Eleazar, who [happened to be] the deputy High Priest. In the words of the Sifre: 123. “‘Scripture here teaches that the Red Heifer was to be prepared by the deputy High Priest. This is made evident [by the fact] that [although] Aaron was still living, Eleazar [who was the deputy High Priest] burnt the heifer. And ye shall give ‘her’ unto Eleazar the priest. This [first heifer] was to be prepared by Eleazar, [“her” — and you shall give ‘her’ unto Eleazar]. Hence Rabbi Meir explains that it means that it was this first Red Heifer that was to be done by Eleazar, whereas all others must be done only by the High Priest. Rabbi Yosei, Rabbi Yehudah, Rabbi Shimon and Rabbi Eliezer the son of Yaakov, however, are of the opinion that the word othah teaches us that only this Red Heifer was to be done by the deputy High Priest, whereas in subsequent generations it may be done even by an ordinary priest. Ramban further on explains the deeper significance of this whole matter. but all other heifers [in subsequent generations] must be prepared by the High Priest.’ These are the words of Rabbi Meir. Rabbi Yosei, Rabbi Yehudah, Rabbi Shimon and Rabbi Eliezer the son of Yaakov say: ‘this one was done by Eleazar [the deputy], and all other heifers may be done either by the High Priest or an ordinary priest.’” The purport of this is to tell us that this commandment, because of its profound secret, deserved to be given to the greatest of the priests, and yet it was not given to Aaron [but to Eleazar the deputy]! Perhaps this was because of his [Aaron’s] greatness, for he was the holy one of the Eternal. and His pious one, who effects atonement in His Sanctuary; therefore He did not want to give him a service which is performed outside [the Sanctuary]. Or perhaps [this was to be done by Eleazar] in order to crown him, and initiate him during the lifetime of his father by means of one of the commandments of the high priesthood. Or it may be [that the performance …