Insert-mode was the default (and only) edit mode for all versions prior to MusiCAD 3 build 594.
Insert mode is active when the button is depressed on the buttonbar.
Any note, whether entered by mouse, keyboard, MIDI buttons or menu is inserted at the cursor position. Insertion in a multi-voice score could thus lead to all kind of problems as the parts would not necessarily line up in time anymore, resulting in a messy display, and troubles in editing. However, sometimes insert-mode is just necessary to get things done correctly/easily.
Overstrike mode is most of the time easier to use, especially when arranging scores or altering melodies.