The SelectNodesWithinRange technique promotes all surfaces from the attached file that lie within the range. That means that the resultant BSplineSurface node is a list of surface geometries, regardless of how many surfaces actually are in the file. This becomes clear when using the surface as a base for the point grid, where the surface must be indexed as a member of a list for the point grid to execute properly. The name of the promoted surface is "bsplineSurface01". Remember, the proper syntax for indexing a member of this list is:

Surface:  bsplineSurface01[0]

In this instance, there is only one surface promoted from the file; there is no consequence if the surface name is indexed or not. However, this must be done if more than one surface is promoted or every surface within the range will be populated by the grid of points.

The MicroStation file scanner searchers for elements in the sequence in which they were created in the original file. Thus, the list of geometry promoted to your GenerativeComponents model is directly related to the order that the geometry was created in the original file. If the original file is edited, this sequence can be altered and indexing of the list in GenerativeComponents can shift.