Transposition in MusiCAD means the number of semitones a part is played higher or lower than notated.
A double bass part is written using bass-clef but actually played an octave lower than its notation, so a transposition of -12 is needed.
An alto-saxophone plays the note Eb when reading C, so it needs transposition +3 to accommodate between reading and playing.
A guitar can be used with a capo to play one or more semitones higher.
Transposition may be most easily accomplished using the part modification dialog with the button