佛頂尊勝陀羅尼經教跡義記 2.55

2.55

唐云最勝清淨也。解曰：虛空寬廣，體喻法身。佛頂最高，名為報應。

In Chinese it means: "Supremely pure." The explanation states: Space is vast and broad, its substance serving as a metaphor for the dharmakāya. The uṣṇīṣa is the highest, named after the saṃbhogakāya and nirmāṇakāya.