The DECT Cellular Cordless telephones are used with the KX-NS1000, TDA, TDE, and NCP series systems (and the discontinued TAW848). They require cell stations, which work in principal the same way as cell phone towers. The cell station you are nearest is the one you get your signal from. When walking through a building with multiple cell stations you will be handed off to the nearest cell station to keep your signal as strong as possible. If there is a spot where you have poor signal and you either can't add another cell station due to port or system limitations, there are signal extenders, KX-A405, that only require AC power.
Panasonic's traditional cordless telephones, the KX-TD7896 and earlier versions, are tied to their own base station and the signal deteriorates the further away you get from the base station. The cellular cordless telephones are not tied to a specific cell station and can therefor roam freely without losing signal, and that is the greatest advantage of the cellular cordless telephones.