ZONE F is a highly helpful tool to check for several plug in values as many a times candidates use very generic/blanket values like positive/negative integers only to deduce an inference.

This can create high chances of errors and such fatal mistakes through sheer negligence can pull down your score.
Plug in values for ZONE F are
 Z- Check with Zero, 
 O-Check with One, 
 N-Check with negative values(integers  and non integers)
 E-Check for extreme values 
 F-Check with fractions