When you select an element in a Revit view, the Properties palette shows a list of all instance properties that are assigned to that element. You can change these values freely, and add your own custom properties.
For details on setting and using these properties, see the Revit help.
When you use Datasmith to import a Revit scene into the Unreal Editor, Datasmith assigns all instance properties for each Revit element as metadata on the Static Mesh Actor that it creates for that element in the Unreal Engine Level. This metadata also includes any type properties for the object's Revit type that are not empty.
Because Datasmith metadata is always a flat list of key-value pairs, the category headings in the Properties palette (such as Constraints, Structural, Dimensions and Identity Data in the image above) are not included. Only the names and values of the actual properties are preserved.