Each map file can contain multiple display maps. Each one has its own set of layers, which are styled independently. For example, you can connect to a data store that contains parcels and then create one display map that themes the parcels by area and another that themes them by population.
If you use the same data store for multiple display maps, you connect to that data store only once, but you add it to each display map separately. You can copy a layer from one map to another and then change its styling.