大毘盧遮那成佛經疏 5.12

5.12

初句言佛子諦聽者，梵云矩羅，是族義部義，補怛羅是男子義。若世諦釋，於四姓中生皆名大族，故名族姓子。今得生如來家，於諸族中最為殊勝，故名族姓子。阿闍梨言：「宜會意，云佛子於義為著也。」

The first line says: 'Child of the Buddha, listen attentively.' In Sanskrit, kula means family or clan, and putra means son. If explained according to conventional truth, being born among the four castes is in all cases called a great family; hence they are called 'sons of good family' (kulaputra). Now, being able to be born into the house of the Tathāgata, it is the most supreme among all families; hence they are called 'sons of good family'. The ācārya said: 'It is appropriate to grasp the sense and say "child of the Buddha", which makes the meaning prominent.'