The modulation order of a digital communication scheme is determined by the number of the different symbols that can be transmitted using it.
The simplest forms of digital modulation are of second order because they can transmit only two symbols (usually denoted as "0" and "1" or as "-1" and "1").
They are called binary shift keying (BSK).
Because existing computers and automation systems are based on binary logic most of the modulations have an order which is a power of two: 2, 4, 8, 16, etc.
In principle, however, the order of a modulation can be any integer greater than one.