EDIT

once an access point has been selected in the fist step it should be possible to limit the search to a specific data set.
Data sets are listed in the web service like for example:

[ 
 { 
   "datasets": [ 
     { 
       "title": "Naturhistorisches Museum Mainz, Zoological Collection" 
     }, 
     { 
       "title": "Naturhistorisches Museum Mainz, Botanical Collection" 
     } 
   ], 
   "service_url": "http://ww3.bgbm.org/biocase/pywrapper.cgi?dsa=NaturkundemuseumMainz" 
 }, 
 { 
   "datasets": [ 
     { 
       "title": "Herbarium Berolinense, Berlin (B)" 
     } 
   ], 
   "service_url": "http://ww3.bgbm.org/biocase/pywrapper.cgi?dsa=Herbar"                                                                                                                                      
 }                                                                                                                                                                                                            
] 

The BioCASE protocol allows to define a data set as filter option. Currently the data set title is being used to identify a data set.