Hi..
The system uses the flows of actual valuesto transfer data from other module to value fields in CO-PA.
You should understand the flows of actual values through the below sap help portal site to find the G/L mapping against value field.
Flows of Actual Values - Profitability Analysis (COPA) - SAP Library
In case of Billing Documents, Values in billing documents are assigned to condition types in SD, accounts in FI and value fields in CO-PA.
You can use t-code KEAT to reconcile the data between FI and CO-PA.
Refer to http://help.sap.com/erp2005_ehp_07/helpdata/en/40/c52df80d1e11d2b5cf0000e82de856/content.htm.
In case of Direct Postings from FI, all assignments of values and quantities to the value fields in costing-based CO-PA are defined using the PA transfer structure "FI“ through the following IMG path: Profitability Analysis->Flows of Actual Values->Direct Posting from FI/MM->Maintain PA Transfer Structure.
In case of Periodic Overhead Allocation, the cost of cost centers is transferred into co-pa using PA Transfer Structure for Assessmentof co-pa cycles.