What does the F in front of brush names in the selector stand for?

This question may seem a little trivial, but it always made me wonder..
What does the F in front of brush names in the selector stand for?

This question may seem a little trivial, but it always made me wonder..
It stands for Fake User. Since the default brushes all have fake users, the "F" shows up in front of the name.
Note: This applies to many other data-blocks, not just brushes, its just noticeable since fake-user is enabled by default, but it can be used used for meshes, curves, actions, fonts... too.