It's multiple entries for a combined total of 90 days over the course of the year according to what the Belgians and Dutch have been telling my Eastern European acquaintances with the one-year multiple entry tourist visas that mention 90 days. That would be a practical limit of 90 total days in a one year period. That said, I'm finding Belgian and Dutch passport control to be all over the place in their claims about what is and is not lawful. It really depends on whether or not the issued visa is a "circulation" visa or not; if it is "circulation" then the practical maximum limit is closer to double 90 days in a one-year period as long as not staying more than 90 days in any given 180 day period.