There are many RPL Predefined functions that reference Subbasins including:

In addition, GetAllNamedBasins returns a list of all the subbasins in the model. See GetAllNamedBasins in RiverWare Policy Language (RPL) for more information.

Finally, the function ListSubbasin returns a list of the objects in the subbasins. See ListSubbasin in RiverWare Policy Language (RPL) for more information.

Within RPL, you can quickly get to the subbasin manager as described in Opening Subbasins From RPL Dialogs in RiverWare Policy Language (RPL)