Provisional custody is a measure ordered by the judge of freedoms and detention at the request of the examining magistrate. The latter may request imprisonment - before judgement - of a person under investigation for a felony or misdemeanour which is punishable by imprisonment of at least three years. Provisional custody must strictly be supported by reasons according to the conditions stipulated by the law.
Source : ministère de la Justice (site : http://justice.gouv.fr)