As per my understanding, the standard feature to default charge account on to PR is based on below logic :
- Charge account specified in the Employee setup form (Human Resources / People / Enter and Maintain / Assignment / Purchase Order Information) for the Requestor
-affected by Expense Account Rules setup in Purchasing Expense Account Rules. Expense account rules can replace one of more of the defaulted charge account segments with a new segment value(s) specified in the expense account rule(s).
As per the project requirement , one segment value should be defaulted from Expense Account rules setup. But in case , if it is not setup , the system should give an error rather than defaulting from the Employee Expense account.