The subtlety of possible objects of concentration ends only at the undefinable.
Having such subtle objects extends all the way up to unmanifest prakriti.
Prakriti means the most subtle matter that there is – Satchidananda describes it as:
the primordial basic substance in its unmanifested condition
Patanjali is saying here that the mind has the power to contemplate the most subtle matter that there is, from the gross through the subtle levels.