As far as I understand, a "perfect unison" refer to two notes sounding the same pitch. Why both MuseScore and Sibelius 7 (I work with these two programs) offer a feature to transpose the up or down by "perfect unison"?
I tried to transpose a single A placed on the usual staff. When I select "augmented unison", the computer places the # in front of it (if to transpose up) or b (if to transpose down). Hence the augmented unison seems exactly one half tone distance.
However nothing happens when I try to transpose by "perfect unison", that makes sense taking its definition into consideration. But why to have such a "feature" at all? Maybe something would happen when transposing more complex piece with constructs I am not aware?