The group that starts with the Surfaces keyword defines the triangular surfaces to be drawn between nodes in the geometry displays. For the building model example the definition is like this:
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This is the surfaces definition group.
From first Node Number, second Node Number, To third Node Number.
Surfaces
2 1 4
2 4 5
3 2 5
3 5 6
5 4 10
5 10 11
6 5 11
6 11 12
8 7 1
8 1 2
9 8 2
9 2 3
11 10 7
11 7 8
12 11 8
12 8 9
3 6 12
3 12 9
4 1 7
4 7 10
This group has three fields per record and as many records as there are surface triangles in the geometry. The three fields indicate the nodes of the corners of the surface triangle. In the example a surface triangle has to be drawn between node 2, node 1 and node 4, between node 2, node 4 and node 5... etc. There can only be one surface definition per record. If a specified node does not exist an error will occur.