это просто посадка прокладки (на ее место для уплотнения)
Цитата VIII-1 Appendix 2-5:
(2) Gasket Seating. The conditions existing when the gasket or joint‐contact surface is seated by applying an initial load with the bolts when assembling the joint, at atmospheric temperature and pressure. The minimum initial load considered to be adequate for proper seating is a function of the gasket material, and the effective gasket or contact area to be seated, ...