investment?

My sister is buying a huge house in New Jersey. The seller said that it’s a mansion but my sister thinks it’s not a mansion. The house has 6 bedrooms and 3 bathrooms. It has a lot of space for a kitchen, dining area, living room, and it has a garage, front yard and backyard. If it’s a mansion, who will pay for the “mansion tax”? Is it the buyer or seller? What is the tax rate of a mansion? What point is a home considered as a mansion?