According to the Mishnah (15b) one of the people whose sacrificial service would be invalid is a mehusar begadim – akohen who is missing one of the four unique articles of clothing that the priests are commanded to wear. The Gemara on today’s daf (=page) seeks to find a source for this law.
Rabbi Avuha quotes Rabbi Yohanan as offering a teaching from Rabbi Elazar ben Rabbi Shimon who suggested that the source is Sefer Shemot (29:9) that teaches that Aharon and his sons should be dressed in these unique garments and that they would serve as kohanim – as priests. This is understood to mean that as long as they wore these clothes they would be kohanim, but without them they would not be able to serve in that capacity. Thus, the sacrificial service that they performed without them is invalid.
Rashi and others understand this statement literally, meaning that without the full priestly attire the kohen who works in the Temple is considered a zar – a “stranger” to the priestly service – whose service is rejected and who will be liable to receive a Heavenly death sentence.
Rabbi Avraham, son of the Rambam argues that we still must distinguish between Temple service performed by someone who is not a kohen and that done by a kohen who is mehusar begadim. For while there is a full negative commandment forbidding a non-kohen from performing the Temple service, the prohibition for a kohen to be mehusar begadim when he performs such service is a negation of a positive commandment – the commandment to wear the clothing so that the service will be done le-khavod u’le-tiferet – for honor and splendor (Shemot 28:2) (see the Rambam‘s Sefer ha-Mitzvot, Positive Commandment 33).
In his gloss to the Sefer HaMitzvot, the Ramban disagrees, and says that wearing the priestly garments is not a separate commandment, but is simply preparation that must be done for performing the Temple service.