From MusiCAD

Import ([File|Import]) is to open and translate non-MusiCAD files into MusiCAD syntax. When importing, a translation is made as best as possible of the data available in the import file.

Importing is possible for:

abc files describe (just like MusiCAD) the notes of the melody. ABC notation is based on the scale used, while MusiCAD stores the notes independently of the scale used. abc is a simplified representation of the notation, while MusiCAD tries to store a (simplified) form of the music.
MIDI files consist of instructions (note-on / note-off) with the corresponding time. The notation must be derived from this. If music is played very freely, this becomes difficult, and the time information will have to be rounded off to get a legible notation. Information is therefore lost when importing.
MusicXML was created to represent sheet music as accurately as possible. Information is lost when importing; MusiCAD only takes note length and height - the formatting (clustering, note spacing, stem length etc.) is largely ignored.

See also