What is the purpose of the Manage Transaction Account Definitions: Project Costing

Summary:

Would like to know when and where the account combination setup at Manage Transaction Account Definitions: Project Costing is applied.

Content (required):

Client has GL Budgetary Control enabled, are using receipt account and are accruing at receipt. When a purchase order is received and the receipt is created, the cost must be transferred to cost accounting, the receipt distributions must be created and then accounted in the GL. The import Costs from Cost Accounting process is run and the receipt fails import into projects due to invalid budget account. Once a generic account combo is added to the Transaction Account Defintion: Project Costing, the transaction is successfully imported into Projects. Want to understand where the accounts assigned here are used and when.