In engineering, especially communications and military, a mobile device or system that can be operate "on the move" (OTM) from a field-deployable vehicle or aircraft, but is not permanently mounted in its carrying vehicle. While general usage may include handheld or vehicle-mounted equipment, formal designation structures, such as the U.S. and allied military AN- designation system, reserves this usage for systems that may move among different carrying vehicles.
Mobile, in this context, is more flexible than transportable, which can be moved in vehicles, possibly after being put into shipping containers, but cannot operate OTM. Handheld, backpack, or other individual devices are considered portable.
Mobile systems and components that are permanently attached to a vehicle, which is not dedicated to the system application, are termed, in military usage, "Vehicle", "Aircraft", "Ship", or "Submarine". For example, a radio permanently mounted in a tank, cabled to its power supply and antenna, would be have a V (vehicle) prefix in the AN- system. If those same electronics were put into a backpack, along with batteries and portable antenna, it would get a P (personal) prefix.
As opposed to the AN- system, which does not have a specific designation for mobile equipment independent of the vehicle carrying it, the Warfighter Information Network-Tactical makes a strong difference between mobile and transportable. Mobile equipment can operate OTM in a truck, tank, or helicopter. Transportable equipment is carried from place to place by trucks, helicopters, boats or aircraft, but requires some assembly before it can operate.