Srimad Bhagavatham : 5.4.10, 11, 12, 13.
Following Bharata, there were ninety-nine other sons. Among these were nine elderly sons, named Kusavarta, Ilavarta, Brahmavarta, Malaya, Ketu, Bhadrasena, Indraspṛk, Vidarbha and Kikata.
Slokam-11 & 12.
In addition to these sons were Kavi, Havi, Antariksha, Prabuddha, Pippalayana, Avirhotra, Drumila, Camasa and Karabhajana. These were all very exalted, advanced devotees and authorized preachers of Srimad-Bhagavatham. These devotees were glorified due to their strong devotion to Vasudeva, the Supreme Personality of Godhead. Therefore they were very exalted. To satisfy the mind perfectly, I [ Suka maharishi ] shall hereafter describe the characteristics of these nine devotees when I discuss the conversation between Narada and Vasudeva.
In addition to these nineteen sons mentioned above, there were eighty-one younger ones, all born of Rshabhadeva and Jayanti. According to the order of their father, they became well cultured, well behaved, very pure in their activities and expert in Vedic knowledge and the performance of Vedic rituals. Thus they all became perfectly qualified brahmanas.
From this slokam we have good information of how the castes are qualified according to quality and work. Rshabhadeva, a king, was certainly a kshatriya. He had a hundred sons, and out of these, ten were engaged as kṣatriyas and ruled the planet. Nine sons became good preachers of Srimad-Bhagavatham (maha-bhagavathas), and this indicates that they were above the position of brahmanas. The other eighty-one sons became highly qualified brahmanas. These are some practical examples of how one can become fit for a certain type of activity by qualification, not by birth. All the sons of Maharaja Rshabhadeva were kshatriyas by birth, but by quality some of them became kshatriyas, and some became brahmanas. Nine became preachers of Srimad-Bhagavatham (bhagavatha-dharma-darsanah), which means that they were above the categories of kshatriya and brahmana.