Here is a mini tutorial about adding User-Data to a scene. The user-data can be used for a lot of things explained elsewhere on this site.
1. Choose a place to add the user-data, this can be a Null or the xpresso tag, or anywhere.
2. Go to the Attribute manager and from the User-Data menu choose add user-data...
3.Choose the type of output the and the name the User-Data should have, there is a range of choices here.
4. Drag the object or tag where he user-data is created to the xpresso manager and start using it for what you need.