Import Overwrites all fields?

Import Overwrites all fields?

Hello,

I was told that if I import records, lets say a table of contacts and accounts, that it will UPDATE The record if it finds the same unique identifier, typically email. Great, but what will happen if..

A blank field is imported , will it overwrite a populated field?
In a MULTI select drop down, if options A-D exist, A was previously, selected, B was imported.. What will remain on import?

And will Notes, tasks, things not imported, be left untouched on a current record?

THx