An alternative way of scaling the alteration between 2 notes accross an octave.
In microtonal notation, the octave is divided in more than 12 steps (dodecatonic).
Some composers base their music on the harmonic series, some divide the octave into 19 or 31 equal parts, some divide the octave into 43 unequal parts, some don't divide the octave at all....
There is an infinite number of ways to choose your tonal resources.