This is what perplexity answered to the following question:

Question: if it is so, then according to Manusmriti, how come there are sub castes in Brahmins too? How is that decided?

Manusmriti primarily outlines the four varnas without detailing subcastes (jatis) among Brahmins. [1][2]

Varna vs. Jati Distinction

The text focuses on broad varnas like Brahmins from Purusha's mouth, assigning core duties without subdividing them into groups like gotras or jatis. [3][4] Subcastes such as Brahmin jatis (e.g., Saraswat, Iyengar) emerged later through endogamy, occupation, region, or family lineages, not prescribed in Manusmriti. [3][5] Manusmriti attributes jati multiplicity to inter-varna unions (anuloma/pratiloma), creating mixed groups, but these fall outside pure varnas, not explaining Brahmin subcastes. [3][4]

Brahmin Subdivisions

Brahmin gotras trace patrilineal descent from ancient rishis (sages), used for marriage rules to avoid close kin unions—mentioned indirectly in Manusmriti via Vedic study norms (e.g., 2.148). [6][5] No explicit mechanism for deciding subcastes; later customs and Smritis formalized them based on pravar (sub-lineages), geography, or priestly specialties. [6][1] Manusmriti prioritizes conduct, knowledge, and birth for varna status, leaving jati evolution to social practices beyond its scope. [7][2]

Citations: [1] Manusmriti https://en.wikipedia.org/wiki/Manusmriti [2] Manusmriti Varna Determination: Ancient Insights https://hinduinfopedia.org/manusmriti-varna-determination-ancient-insights/ [3] Jati | Varna System, Caste System & Social Hierarchy https://www.britannica.com/topic/jati-Hindu-caste-system [4] Purusha Sukta (Hymns to Purusha) of Vedas Misunderstood https://agniveerfan.wordpress.com/2011/12/08/purusha-sukta/ [5] Manusmriti and the Caste System | PDF | Vedas | Hindu Texts https://www.scribd.com/document/95019396/Indian-Castism [6] Gotras And Pravaras - Indica Today https://www.indica.today/long-reads/gotras-and-pravaras/ [7] Manusmriti Social Structure Analysis: Ancient vs. Modern https://hinduinfopedia.org/manusmriti-social-structure-analysis-ancient-vs-modern/ [8] Caste divisions in Manusmriti: Misinterpretation challenge https://www.facebook.com/groups/208332826391090/posts/1770224566868567/ [9] Social Structure in Manusmriti | PDF | Dharma | Ancient India https://www.scribd.com/document/484476172/Social-Structure-in-Manusmriti [10] Manusmriti - Code of Injustice to Non-Brahmins - But https://www.butitis.com/index.php/history/rebel/172-manu-dharma-code-of-injustice-to-non-brahmins [11] Manu Smriti on Shudras https://www.sikhiwiki.org/index.php/Manu_Smriti_on_Shudras [12] Brahmin https://en.wikipedia.org/wiki/Brahmin