How to derive invoice distribution based on project type and attribute on task number

Summary:

We have a requirement to derive certain segments of charge account on AP Invoices for project based transactions on the basis of Project Type and a DFF segment on project header and task DFF. The user will only select project , task , exp type, exp org and exp item date. The expectation is that there is an SLA rule which will derive the necessary based on project and task selected. However, when i create a mapping set using project billing as a source (which is where i can select project type, DFF etc) and use this in the