What is "airspace underlying a Class B airspace area designated for an airport"?
Yes, there is a legal interpretation on this:


  [...] under  the  express  language  of  the  regulation,  aircraft 
  operating  in  the  airspace  underlying  Class  B  airspace -
  irrespective  of  whether  the  underlying  airspace  is  controlled 
  or  uncontrolled - may  not  exceed  the  200-knot  speed  limitation


So it is indeed any and all airspace underlying class B.

And note the exception to the rule:


  We  note  § 
  91.117  (d)  does  provide  that,  if  the  minimum  safe  airspeed  for  any  particular  operation  is  greater  than  the  maximum 
  speed  prescribed  in  § 
  91.117,  then  an  aircraft  may  be  operated  at  that  minimum  speed.