Digital BW, The Print

I posted a message here yesterday asking what I thought was probably 
one of the simplest questions asked here recently, i.e. I asked what  
the abbreviations "OFB" and "PFA" in the GUI Curves drop down menu 
stood for. I received no responses. I can't believe the lack of a 
response means that nobody here knows what papers these abbreviations 
stand for so I'm asking again. Surely there's at least one person 
here who knows the answer. Thanks.