Field status determines how a field is displayed or 
maintained. 
Field status variant is a collection of field status group.
Field status group means -
a) Combinition of field statuses for a group of fields.
b) Defines the screen layout at the time of document entry 
at line item level.
c) Assigned to each G/L A/C.

Conclusion:- Combination of field status groups maintained 
in field status variant.

Plz, let me know anything more if i am wrong? to all sap 
gurus.