The MK3.1 has an extremely powerful 2.4GHz FHSS radio, which is certified at right up to the max allowed for FCC, CE certification.
We quote the "up to 1.5Km" range, because that figure is directly comparable to what other radio devices quote, but it is worth explaining what that means in the real-world.
To be clear, the figure is pretty optimistic, by definition. In perfect conditions you can achieve this distance, but you can not assume (or plan for) perfect conditions.
Many things reduce wireless range- obstacles, metal objects, reflections (multipathing) from architecture and landscape, antennas angle (polarisation), interference. FHSS radios (like what we use) are the best at punching through all of that and retaining a good signal, but range still decreases eventually.
Clearly this is not an exact science, but when planning for a shoot in a hostile environment, you should probably divide manufacturer ranges by 3 unless your line of sight is excellent (for example aerial footage), or divide by 8 if your signal needs to be absolutely perfect and you know you're going to be contending with a lot of big obstacles and interference.
Please Note: The MK3.1 wireless Range/Power is set to LOW by default, please remember to adjust this wireless power level for the range you need, so for the highest Range/Power use USA MAX or if you are going to operate close to the camera use LOW.