Defaulting Req Charge Account based on Project Information entered

Summary:

We have a requirement to derive one of the segments in our COA (not company or cost center) based on the expenditure organization entered on the PO Requisition Form.

Tried customizing the transaction account rules in Purchasing but we could not find a source for expenditure organization.

We created a mapping set with input as expenditure organization id and mapped the Organization IDs to a segment in COA as output. But we are unable to use the mapping set in the TAB rules. It is giving us activation errors. The error says: Source is invalid for account type charge account.