Floor correlation can be set via the properties of a building in your map.
You can enable correlation between floors in the same building. This is especially useful if you only have one access point per floor. In order to perform localization by triangulation, three access points are needed.
If you have only one access point per floor, three floors are needed to perform localization. And in this scenario, correlation between floors is mandatory to perform localization.
But lets say that you do have three floors, each with three access points. Localization can be performed per floor, i.e. without enabling correlation between floors. In this cenario, enabling correlation will enhance the localization process, because all nine access points can be used to perform localization instead of just the three access points per floor.
So, we advise to enable correleation between floors whenever possible, to allow for more accurate localization.