As we learned in the Mishna on yesterday’s daf there are some korbanot that must be burned entirely after their blood is sprinkled on the altar and their innards are sacrificed. The Mishna teaches that such sacrifices were placed on poles and were carried by kohanim front-and-back to the beit ha-deshen – the ash-pit outside of the city of Jerusalem. According to the Torah (see Sefer Vayikra 4:25; 16:27-28), the clothing belonging to the kohanim who had this responsibility became tameh and had to be immersed in the mikveh.
The Mishna teaches that the clothing of the kohanim became tameh immediately when each one of the kohanim carrying the sacrifice left the walls of the Temple compound. Thus the kohanim who were in front carrying the sacrifice became tameh even though their colleagues in the rear were still tahor – ritually pure. Once all of the kohanim had left the Temple compound, they were all tameh.
The Gemara on today’s daf discusses the source for this halakha and quotes a baraita that explains that regarding the sacrifices brought on Yom Kippur the Torah sounds as if they must be removed and burned outside of a single encampment (see 16:27), while regarding other such sacrifices the requirement is to burn them outside of three camps (the inner camp of the Tabernacle, the middle camp of the tribe of Levi and the outer camp of Israelites) in the desert. From this we understand that although the sacrifices must be burned on the beit ha-deshen outside of all three camps (and in Temple times, outside the walls of the city of Jerusalem), nevertheless, the ritual defilement of the clothing of the kohanim associated with this service takes effect immediately upon leaving the encampment of the Tabernacle (or the Temple courtyard).