A structured, shared reference of standardized property names and definitions - so that "fire rating" means exactly the same thing to every software and every party using the model. Without a shared data dictionary, one tool's "FireRating" and another's "Fire_Rating_Class" describe the same property under two different names, breaking any automated check or handover. buildingSMART's own bSDD is the best-known public example, mapping classification systems like Uniclass and OmniClass to a single online reference.