This is one of the needed items to have in your parts kit. A male to male foster fitting will allow you to move air from one tank to another when used with two fill adaptors. This is helpful if you are trying to move air in a big bottle to a smaller more portable bottle. Simply connect each fill adaptor to the male to male foster fitting. This is also helpful if you are trying to get a SCBA tank filled, but the people who are trying to fill your tank dont have a SCBA Fill Adaptor to connect to your bottle. In that case you can ask if they can fill using a paintball tank fitting. Then simply connect the male to male fill adaptor to the end of your fill adaptor (if it is rated for 4500 psi) and let them connect to that and fill your tank.
rated 6500 psi