1. What are F-Numbers? 

The F-Number System is the American Concrete Institute (ACI 117) and Canadian Standards Association (CSA A23.1) standard for the specification and measurement of concrete floor flatness and levelness.  F-Numbers replace the familiar "1/8th inch in ten feet" type specs that had proven unreliable, unmeasureable and unrealistic. 
The new standards include two F-Numbers: 
                                F_F  for flatness and F_L for levelness   

Flatness relates to the bumpiness of the floor, while levelness describes the local tilt or pitch of the slab. The higher the F-Number, the better that characteristic of the floor.  F-Numbers are linear, so an F_F 20 is twice as flat as an F_F 10, but only half as flat as an FF 40. 
Slabs-on-grade are usually specified with an F_F number and an F_L number (the F_F is always listed first), such as : F_F 25 / F_L20 

Because of deflection, elevated slabs are usually specified using only F_F.  

When a floor is described as an "F 25", it usually means "F_F 25". 
The ACI/CSA F-Number System applies to 99% of all floor slabs - all floors that support random traffic, be it vehicular or pedestrian traffic 

In the tiny percentage of floors that have defined traffic, where vehicles are restricted in their movement by wire or rail guidance, a different F-Number - Fmin - is used.