Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Shell
SOLIDIFYGENERATOR_SELECTION_POLYGON_BOUNDARY
Parameter: Start Cap
SOLIDIFYGENERATOR_SELECTION_POLYGON_SOURCE
Parameter: End Cap
SOLIDIFYGENERATOR_SELECTION_POLYGON_CAP
Parameter: Start Shell
SOLIDIFYGENERATOR_SELECTION_EDGE_BORDER_SOURCE
Parameter: End Shell
SOLIDIFYGENERATOR_SELECTION_EDGE_BORDER_CAP
Parameter: Start Cap
SOLIDIFYGENERATOR_SELECTION_VERTEX_SOURCE
Parameter: End Cap
SOLIDIFYGENERATOR_SELECTION_VERTEX_CAP