Clefs with octave indicators indicate that notes are played in a different register to the one notated. An octave indicator above the clef indicates that notes are played higher than notated, while an octave indicator below the clef indicates that notes are played lower than notated.
Of these clefs, only the treble clef 8 below is still commonly used for tenor vocal parts.
Traditionally, clef octave indicators were used as a reminder of transposing instruments; however, in more recent music, some composers use clef octave indicators as an alternative to octave lines for extended passages. Therefore, Dorico Pro ignores clef octave indicators by default. However, transposing instruments are always transposed correctly automatically. For example, notes belonging to piccolo instruments are automatically notated an octave lower than their pitch in playback, regardless of whether they have a clef with an octave indicator. You can change whether Dorico Pro respects or ignores clef octave indicators in each flow independently.