An Open Lever, also called a Regular Lever, is used for non toxic gases. It is normally used on conventional SRV's and hence open to atmosphere. When the valve opens, fluid escapes via the cap. A Packed Lever is a "gas tight" lever design used for liquids, toxic fluids or when a superimposed back pressure is present in conventional design SRV's (a bellows would shield the cap).

There can be a noticeable difference commercially between the 2.

ASME VIII "UV/NB" Code Stamped SRV's require a lever as a mandatory requirement when the fluid is air, steam or water above 140F (Unless specific rules are followed and accepted).

Per ISO, only the term Safety Valve is used regardless of application or design.