Best bet is to buy the same part you now have, but installing one that has features you don't have (fingerprint reader, etc.) should not be a problem, either.  
so, if i want to replace a palm rest of a Latitude 5590 computer, can i just go on ebay and find one from the same model, or do I have to be more specific.  it seems like there are different models of latitude 5940 (fingerprint reader, etc).  will my motherboard "recognize"  any of them?  if not is there a chart on which models are compatible.   as a rule of thumb, if the ribbon cables and jacks (like power plug, USB port, net work port) line up in the same spot), should that palm rest work for me? THanks
replacing a cracked palm rest on a latitude 5940