You specify that you wish to be able to continue the simulation you are about to execute from the Hot Start dialog in the Hydrodynamic Parameters section, see Hot Data section in the Reference Manual. In other words, you cannot specify separate hydrodynamic and advection-dispersion hot data.
You specify that you wish to do a simulation as a continuation of a previous one by selecting Hot start in the Bathymetry dialog, and then writing the name of the hot data created earlier.