If I understand correctly the perspective from which you are asking this question, market center is right between the buyers' market (more inventory for fewer buyers) and the sellers' market (less inventory for more buyers). National Association of Realtors and it's local branches measure market activity on a regular basis. Every month they publish how many active listings there are in each market for an active buyer. Typically, buyers' market occurs when there are 7, or more homes available per buyer, sellers' market is when there are 5, or fewer, available homes per buyer. So market center would right between the two.
I hope that helps.