Vacuum valves are used to control flow and are connected to the vacuum system with KF "NW" flanges.
Needle and metering valves can be used to relieve the vacuum safely or to control the flow of gas into the system. Block valves are mainly used to open or close the flow and cannot regulate the flow as precisely as metering or needle valves.
Metering valves are needle valves but include markings to gauge the flow. A flow meter and vacuum gauge must be used to quantify those values. The markings will not tell you directly how much flow and are arbitrary because the flow will be depended on the vacuum level.