What is the difference between "Expiration" and "Firm Expiration" in bpminlicense?

Snippet from when I run "bpminlicense -verbose"

  Expiration      = Not expired Sat Feb 16 00:00:00 2013
  Time Left       = 2 Days
  Firm Expiration = Not expired Thu Feb 28 00:00:00 2013
 

What is the difference between the two? There is nothing in the publications or any knowledge article that explains what these fields actually mean.