
This is what letterboxers use to keep track of their boxes. There are more complicated systems, but this is the one that works for me.
F is the number of boxes that you have found. I have found 6 letterboxes, 1 PLB and 1 HH for a total F count of 8. Again some people separate the PLBs and HHs.
X is the number of exchanges you've had. If you meet another letterboxer and swap signature stamp images, this counts as an X. As you can see, I've only exchanged once.
Some people also keep track of Es. This is for Letterboxing events or gatherings you have attended. There is usually a stamp made for the event that everyone present gets an image from.