Yoma, Chapter Two, Mishnah Four
This mishnah teaches what the third and fourth counts were for.
1) The third count: New [priests] come up and submit to the count for the incense.
2) The fourth count: New and old priests, who will take up the limbs from the ramp to the altar.
Section one: The chief of priests would make this statement, directed at the new priests who had never before had the chance to offer the incense. Offering the incense was considered by the priests to be the best task in the Temple because of Deuteronomy 33:10-11: They shall offer You incense to savor Bless, O Lord his substance and favor his undertakings. The blessing and the favor went to the one who offered the incense. In other words, offering the incense would bring a person reward. Since this was so desirable, they let only the priests who had not already performed this task participate in this count.
Section two: The final count was open to both new and old priests alike. The count was to determine who would bring the limbs up and sacrifice them on the altar. Some commentators explain that nine priests would bring the nine items up (see the list in yesterdays mishnah), while other commentators explain that one priest would bring all nine items up himself.