yes i think it should be a filegroup but not sure
i tried this code :
RESTORE DATABASE Bill2007
FILEGROUP = 'january2007'
and the error i recive :
Server: Msg 3135, Level 16, State 2, Line 1
The backup set in file 'D:\DOWNLOAD\Bill_012007.bak'
was created by BACKUP DATABASE...FILE=<name> and cannot be used for this restore operation.
Server: Msg 3013, Level 16, State 1, Line 1
RESTORE DATABASE is terminating abnormally.