The top 16 pairs are for the Channel inputs (Channel 1 through Channel 16, Top to Bottom).  The 17th pair (bottom pair) goes to a 2 x 2 Jumper *(J2) just inside the front panel on all (ADC,QDC,TDC) modules.  This (J2) jumper can connect these pins to the modules ground.  This way, you can connect ground from the driving source to the module (should you need to) by installing these jumpers.  Modules are shipped without jumpers in this location, therefore the 17th pair is floating.  

* For the 7187H, the 2 x 2 jumper is "J2" for the "STOP" header and "J71" for the "START" header