Define your account codes as valid values for this segment. If you use Oracle General Ledger, you should use the same set of values when you define the GL flexfield called Accounting.
Suggestion: If you are not using Oracle General Ledger you could set up your own table of financial codes and values and use Table Validation for your cost headings. You can set up a table in the Table Structure window.
You could also consider populating this table from your existing financial systems.
See: User-defined Tables
If you want to do double entry accounting, you should also set the qualifier Balancing to Yes. This allows you to enter two account codes for each element link: one to credit and one to debit. You enter these codes in the Costing and Balancing fields in the Element Link window.