Value Field in COPA reports

Hi Experts,

We have a requirement to create Sales Report using a form and KE30 transaction. I am able to copy a form and create a report. But, can any one brief me on the logic how the data is being fetched from Value fields. The value fields are linked to G/L,s ?. Where I can find value field & G/L related data?. I am interested in knowing the logic behind these reports.

Thanks in advance.

Mohan