Class I and VI fittings and class VI flanges. - Class I and VI fittings and class VI flanges are intended for use only where fittings and flanges with special dimensions and tolerances not covered by class II, III, IV and V fittings and flanges are required.

Class II fittings. - Class II fittings are intended for applications in piping systems utilizing tube having dimensions comparable to class II of MIL-T-23227.

Class III and V fittings and class V flanges. - Class III and V fittings and class V flanges are intended for applications in piping systems utilizing pipe with standard commercial dimensions conforming to the table 2 of ASA B36.10 and class III of MIL-T-23227.

Class IV fittings.- Class IV fittings are intended for applications in piping systems utilizing pipe with a controlled ID to facilitate consumable-insert welding which, from a design consideration, is interchangeable with pipe conforming to table 2 of ASA B36.10 and class IV of MIL-T-23227.