The substratum is that which underlies past, present, and future characteristics.

Śānta is past, what is quieted. Udita is present, what has arisen. Avyapadeśya is future, what is not yet manifest. Dharma are characteristics. Anupātī is following, underlying. Dharmī is the substratum.

Behind all changes there is something that remains: the dharmī, the bearer of dharmas.

Clay remains even though the form changes from lump to pot to shards. The substratum persists through transformations.

This sutra has profound philosophical implications. The dharmas (characteristics) change, but the dharmī (substratum) remains. It is the Sāṃkhya-Yoga answer to the problem of change.

For the yogī, this means there is a stable reality behind changing appearances. That reality can be known through saṃyama.

The puruṣa is the ultimate dharmī: unchanging while prakṛti unfolds its infinite transformations.