Any MusiCAD song will need some kind of reference point, signaled with a clef.
By far, most music is written using the treble clef which is default. When creating a new song you will be asked to choose from a list of possible song templates. This will include the clef(s) as well.
Most of the C-clefs (soprano, mezzo-soprano, alto, tenor, baritone) aren't much in use nowadays. The alto clef (just in between treble clef and bass clef) is still in use for the alto-violin.
Instead of using some C-clef, an octave selection is used using 8va or 8va basso. MusiCAD will handle these the same way as an ordinary clef.
A special kind of clef is the neutral or percussion clef, not denoting any special pitch but just indicating that one or more percussion instruments ar te be played.
If you need to alter or insert a clef-change you'll use the clef menu.