Terrain extractor interpolates their elevation by first finding the face that the point is on? How do you do Planar Interpolation?

I called Bentley and they told me that Openroads uses terrain extractor to interpolate elevations inside the triangles on a digital terrain model to create a surface, but I don't understand there method...
Point Coding Example

Product(s): OpenRoads Deisgner, OpenSite Desginer Version(s): 10.10+ Area: Survey Question: Can you provide an example of the linking codes available in OpenRoads Survey?...
Point Coding example files

https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/16136/point-coding-example
Interface point Annotation Video

Description of a workflow to annotate "interface points", identifying the point where the tie slope changes from Fill to Cut / Cut to Fill Sample Dataset : https://communities.bentley.com/products/road___site_design...
Interface Point Annotation Dataset

Sample Dataset to annotate "interface points", identifying the point where the tie slope changes from Fill to Cut / Cut to Fill