Comments

Comm. NT: 19.
Catudhātuvavatthānabhāvanāvaṇṇanā

‘By characterizing individual essences’: by making certain (upadhāraṇa) of the specific characteristics of hardness, and so on.
For this meditation subject does not consist in the observing of a mere concept, as in the case of the earth kasiṇa as a meditation subject, neither does it consist in the observing of the colour blue, etc., as in the case of the blue kasiṇa as a meditation subject, nor in the observing of the general characteristics of impermanence, etc., in formations, as in the case of insight as a meditation subject; but rather it consists in the observing of the individual essences of earth, and so on.
That is why ‘by characterizing individual essences’ is said, which means, ‘by making certain of the specific characteristics of hardness, and so on.

пали	Tattha vavatthānanti sabhāvūpalakkhaṇavasena sanniṭṭhānaṃ, catunnaṃ dhātūnaṃ vavatthānaṃ catudhātuvavatthānaṃ.
Nyanamoli thera	Herein, “defining” (vavatthāna) is determining by characterizing individual essences. 19 [The compound] catudhātuvavatthāna (“four-element defining”) is [resolvable into] catunnaṃ dhātūnaṃ vavatthānaṃ (“defining of the four elements”).