Something I can answer! Even if it is inconsequential ...
BF is a common civil service system - what happens is you write to someone, decide you should a reply within say 4 weeks and check a chart. This chart tells you what coloured tags for files will mean a clerical officer will pull the file out for you and bring it to you. 4 different colours for varying the length of time it is put away in the filing racks. Of course it is probably digital system now? But that is was BF is. And it meant brought forward .