[mahul]

Here's my take

No energy is analog or digital, its a device that is analog or digital.

An analog device accepts the voltage value as it is, while a digital device makes some decision about the signal.

A digital device can only work with a fixed set of values. For example a microcontroller( and most other devices ) works with just two values, 0 and 1. So whatever voltage is supplied, it compares the voltage with a reference. If the value is higher than the reference, it considers it as 1, else 0. Now your q would be easy to answer. If you consider say, 2.5 volts as the reference, 3 and 5 V is '1' and 0 and -3 is '0'. The choice of the reference depends entirely on the designer.

CEans, please point out errors if any.