What is mds_stores process?

My iMac was making quite a bit of a rumbling noise and I noticed, using Activity Monitor, that mds_stores was using up 50% of CPU processing power.

I read that mds is linked to Spotlight indexing, but mds_stores was active while no indexing was going on.

Does anyone know what mds_stores purpose is, i.e. what it's use is or what it does?