From among the noble disciples practicing under any given Buddha, a select number are picked by that Buddha as preeminent in a particular field. These are the great disciples. For example, each Buddha appoints two as chief disciples (Pali: aggasāvaka). During the Buddha Gotama’s time, these posts were filled by Sāriputta (the foremost in wisdom) and Mahāmoggallāna (the foremost in exercise of psychic powers). Other posts include guardianship of the Dhamma and the foremost in doctrinal exposition (assigned to Ānanda and Mahākaccāna respectively during the Buddha Gotama’s time).
But it is important to see that these posts are not given to noble disciples by choice. The Buddha who assigns these posts is confirming the fruition of an early aspiration made by that noble disciple in the deep recesses of their past.
Each great disciple attains such a position following a similar pattern. During the time of some Buddha in the past, a follower of that Buddha witnesses him proclaim some disciple as preeminent in a particular field. Instead of striving for arahantship in their own lifetime, this follower “forms an aspiration (Pali: patthanā, abhinīhāra) to attain, under a future Buddha, the same post of preeminence.”
 I here refer to the Buddha of some past time with ‘him’ because as far as I know there is no instance of a female Buddha recorded in the Theravādan tradition.
 Great Disciples, p. XXIV.