Comma-separated related objects to return in full instead of as IDs. Accepted values: account, account.parent, branch, contact, contact.persons, cost_center, item, journal, project, tax_rate. Nested relations are joined with a dot. Pass journal to replace the journal ID with its object (ID, date, currency, reference, notes and timestamps). Documents without a journal still return null. Journal line items are retrieved separately with GET /v1/journal-line-items/?journal={journal_id}.