One common (though not exclusive) use of "ficta" was to avoid harsh harmonic or melodic intervals such as the tritone, for example the use of a B-flat instead of a B-natural to avoid dissonance with an F in another part.
One common (but not exclusive) use of "ficta" was to avoid harsh harmonic or melodic intervals (for example the tritone, the'diabolus in musica'). An example might be the use of a B-flat instead of a B-natural to avoid a tritone against an F in another part.