Is one app or one application equal to one device, if I can at maximum activate 6 devices with ADE?

Adobe tells me, that I can activate 6 devices with my Adobe-ID for Adobe Digital Edition (ADE). I have four devices - 2 smartphones, 1 tablet, 1 computer - in total, but on installing an additional eBook reader app on my smartphone (in total now 4 eBook Reader apps), I got the message, "you reached the maximum count of available activations for ADE". So my question: is one app or one application equal to one device and consumes each app or application a license for ADE, regardless the count of physical devices (with maximum 6).