Hi,
Frequency is defined based upon the infotype and client requirement.
in Infotype 14 you can deduction for specific period by defining startdate and end date. Like client wants to deduction Mobile bill recovery flat 500 from April to June 15. Then you will maintain 01.04.2015 to 30.06.2015 as a start date and end date.
But suppose want to deduct 600 on the July month then you can maintain record in the IT14 as well as IT15. In the IT14 you will maintain as 01.07.2015 to 31.07.2015 and in the IT15 you will maintain any of the date of the July month2015.
Check and tell me.
Regards, Sankarsan