When applying for an academic position, usually you provide a list of names of people who can give you a reference or letter of recommendation e.g. postdoc advisor or someone you often collaborate with. In this case, obviously you first ask the person if they are willing to do this.
However, for applications to academic positions in some countries, e.g. Switzerland, there's a different system. One is asked to provide the names of potential external referees, i.e. people who work in the same field as you and can evaluate the quality of your scientific output. These are people who don't necessarily know you personally, i.e. the kind of people you would suggest as potential reviewers for one of your articles or grant applications.
My question is this: in the latter case, is it usual or necessary to contact the people before suggesting them as referees? Obviously you wouldn't do so for an article or a grant application, but what about when it's for a job application?