Why warning display Create an Unlinked Terrain Model

Create Terrain Model from Elements in open Roads Technology, Civil Tools > Terrain model > Create from Elements, warning display “Create an Unlinked Terrain Model”:

Cause:  The terrain model is created and ruled to the graphic elements. Thus, edits to the graphic immediately update the terrain model. Using this option on large datasets takes a significantly longer time to build the terrain model. The DGN is scanned and the first 15,000 elements found are added to the rule that is attached to the terrain model that gets created. Changing one of these elements affects the terrain model. All other elements found in the DGN are added into the terrain model but not added to the rule. Changing one of these elements does not affect the terrain model. Hence warning displays in the Civil Message Center if the limit is reached.