These thumbscrew core depressors are designed to be used on the end of a charging hose. When attached to an access fitting, they can depress the valve core and control the flow of gas. This also means the core can be shut off before disconnecting, thus avoiding a spit back on your hand. The seals in each part have broad compatibility with all
refrigerants. The CD2070 is unique in that it has an internal shut-off  that also holds gas in the hose when disconnected.