Under all six substances, one definition: a thing exists if it originates, ceases and persists - all three at once. The pot is made and the lump of clay is gone and the clay remains.
Utpāda-vyaya-dhrauvya-yuktaṃ sat. Nothing that is real is only permanent or only changing; every substance keeps its nature while its states arise and pass. Gold stays gold while the bangle becomes the coin; the soul stays the soul while it is a god and then an ant; space stays space while what is in it moves. This is the sentence the whole doctrine of many-sidedness, anekānta, is built to protect, and the reason the six can be eternal and the universe can still have a history.
It is also why the six cannot be reduced to fewer. Each keeps a nature none of the others has - knowing, form, carrying motion, carrying rest, giving room, marking change - and each keeps it through every state. The count is six because that is how many natures there are.
The six, compared as the sūtra compares them - What it means to exist highlighted