Comma-separated related objects to return in full instead of as IDs. Accepted values: account, account.parent, attachments, branch, contact, contact.persons, journal, paid_through_account, paid_through_account.parent, project, tax_rate, cost_center. 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}.