Invoice Planning

hai ,

At present we are in new implementation ,my client have a godown (on rental) and they are paying the rent on 5th of every month .so we plan for periodical invoice planning .

So i gone through the SAP Help .but i didnt able to understand the control datas like

u2022 Horizon,

u2022 Dialog purchase order

u2022 Maintain the date category

kindly explain the partial invoice planning customization settings in breif.it will be more useful for our ongoing project.

thank you

with regards

Peter .