
The key of F has one flat (B flat). This is opposite of the key of F sharp which the B is the only natural (all of the other tones are sharps).
If we view F (really F/C in the grid above) as offspring of the E form, this fingering of F is the inside 4 strings of the E form. It is common to use this type of fingering in combination with the type of C voicing we've chosen here (C/G).