They are filed by someone who is not a party in the case but who wants to weigh in with information or arguments that the case might consider. Can be a person or an organization or government entity. For example, the American Civil Liberties Union often files briefs on behalf of a party who contends his constitutional rights have been violated, even though the claimant has his own attorney. Friends of the Earth or the Sierra Club may file a supporting brief in an environmental action in which they are not actually parties. Usually the court must give permission for the brief to be filed and arguments may only be made with the agreement of the party the amicus curiae is supporting, and that argument comes out of the time allowed for that party's presentation to the court.