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