After the root bhū 'to be', in construction with a case-inflected word as its upapada and when used without a preposition, the affix kyap is used to denote condition - bhāva,
The words सुपि and अनुपसर्गे of previous sutras are understood in this. The anuvritti of यत् does not cover this aphorism. As ब्रह्मभुवं गतः = ब्रह्मत्वं गतः 'gone to or arrived at Brahmahood'; देवभुवं = देवत्वं गतः 'gone to divinity'.
The word bhava of this sutra governs the succeeding ones.
Why do we say 'in construction with a case-inflected word'? Otherwise it will take यत्; as भू + यत् = भो + यत् = भव्यम् (VI. I.79).
Why do we say 'not taking a preposition'? Observe प्रभव्यम् ।,