Multi user is a term that defines an operating system that allow concurrent access by multiple use of a computer most of the time shared system are multi user system also most of the batch processing system are multi user system that does not make the cpu idle .

DOS is single tasking as you cannot open multiple programs at a time.Plus it is for single-user as you cannot make multiple user profiles on it and everything is open for everyone.

Spoofing --->spoofing is a method of attacking computer program in which the program is modified so as appear to be working normally but in reality it has been modified with the purpose of circumvent security.