The chromatic semitone, in 31-ET, is one step smaller than the diatonic semitone. It is sometimes called an augmented unison.
The chromatic semitone sounds more dissonant than the diatonic semitone, but less dissonant than the diesis. Unlike the diatonic semitone, the chromatic semitone is almost always dissonant, and is rarely or never used as consonance. Also, unlike this interval, the chromatic semitone does almost nothing in terms of implying a tonal center.
Melodically, in western music this interval usually only plays the role of moving between other more musically-important intervals. However, 31-ET opens up the possibility of new melodic uses for the chromatic semitone, as it can now be distinguished from both wider and narrower intervals smaller than a whole step.