The 30A connectors are recommended for most uses, including power connectors on radios, and accessories such as TNCs, computers, small inverters and the like, as well as on power sources such as batteries and power supplies. These are the most commonly used.
The 75A connector is expected to be used where either high current devices are used (such as high-power inverters), or other applications where heavy gauge cable needs to be terminated, such as the heavy-duty 50A or 100A flexible cable sold by various outlets. Such an application could be a long cable run from a 12V battery to a breakout box located in a tent, for example.
Where 75A connectors are used on power sources, it is recommended that one or more 30A connectors also be provided to enable appliances to be connected directly to the power source without needing to find an intervening adaptor cable or breakout box.