This tool allows you to add classifications to objects directly in Revit.
When you click on the "Classify" button in the plugin, a window opens. It allows you to select objects and associate classification nodes to them.
Your Onfly platform is probably set up to use multiple classifications. All these classifications can be used in Classify as well. The dropdown menu allows you to select the classification you want to use.
When associating a class to an object, it is possible to download the recommended properties for this class. This is why, when clicking on a class, the list of recommended properties is displayed in a new pane on the right.
To associate a class to an object, select objects via the checkboxes in front of theirs names, then double-click on the class name.