Goal: Given a motor model with a named selection of faces and two named selections of vertices, all of the same size, for each of the three lists, add a coordinate system at the center of the face with its +Y axis pointed out of the face. The -X direction will be from the vertex in the second list to the vertex in the third list.

Code:

# Get all faces and vertices of named selections
face_named_selection = Model.NamedSelections.Children[0] # First named selection, of faces
face_ids = face_named_selection.Ids
start_vertex_named_selection = Model.NamedSelections.Children[1] # Second named selection, of vertices
start_vertex_ids = start_vertex_named_selection.Ids
end_vertex_named_selection = Model.NamedSelections.Children[2] # Third named selection, of vertices
end_vertex_ids = end_vertex_named_selection.Ids

# Create axis systems
selection_info = ExtAPI.SelectionManager.CreateSelectionInfo(SelectionTypeEnum.GeometryEntities)
with Transaction(): # Suppress tree update for performance
    for iFaceCount in range(0, face_ids.Length):
        coordinate_system = Model.CoordinateSystems.AddCoordinateSystem()
        selection_info.Ids.Clear()
        selection_info.Ids.Add(face_ids[iFaceCount]) # Create a temporary selection

        # Define origin in face center
        coordinate_system.OriginLocation = selection_info

        # Define primary axis (Y) normal into face
        coordinate_system.PrimaryAxis = CoordinateSystemAxisType.PositiveYAxis
        coordinate_system.PrimaryAxisDefineBy = CoordinateSystemAlignmentType.Associative # Geometry selection
        coordinate_system.PrimaryAxisLocation = selection_info

        # Define orientation around principle axis from start- to endvertex
        selection_info.Ids.Clear()
        selection_info.Ids.Add(end_vertex_ids[iFaceCount]) # Create a temporary selection
        selection_info.Ids.Add(start_vertex_ids[iFaceCount]) # Create a temporary selection
        coordinate_system.SecondaryAxis = CoordinateSystemAxisType.PositiveXAxis
        coordinate_system.SecondaryAxisDefineBy = CoordinateSystemAlignmentType.Associative # Geometry selection
        coordinate_system.SecondaryAxisLocation = selection_info