Multipartite viruses have a dual personality. Some are file viruses that can also infect system sectors; others are system sector infectors that can also infect files.
Some viruses can be all things to all machines. Depending on what needs to be infected, they can infect system sectors or they can infect files. These rather universal viruses are termed multipartite (multi-part).
Sometimes the multipartite virus drops a system sector infector; other times a system sector infector might also infect files.
Multipartite viruses are particularly nasty because of the number of ways they can spread. Fortunately, a good one is hard to write.
Multipartite viruses have dual capabilities and typically infect both system sectors or files.