We call mods the battery pack that houses the power batteries and the electronics that control the power output, voltage, current, provide protection, charge the batteries (if that is the model). Very often there is an LCD display for indication and a USB connector for charging, often for firmware upgrades. Buying with the evaporator is most often done with a type 510 connector in which the evaporator is screwed. The connector is standard, as are the standard and corresponding threads of the evaporators - so that all evaporators can be screwed in all modes (of course there are exceptions caused by the design)