Substitutes are nominated, not at the time the vacancy arises but, rather, before the election for the information of voters.
For municipal and provincial elections, as well as those for the Parliament of the Brussels-Capital Region since 2019, there is only one list, and substitutes are designated, according to various systems from one region to another, on the base of their own preferential votes, weighted or not with list votes, i.e. not for one or several specific candidate(s).
The substitute assumes the functions of the deputy if the deputy dies, enters the executive government, is appointed by the Government to an assignment of more than six months' duration, or appointed to the Constitutional Council or Defender of Rights (Défenseur des droits).
[1] If the deputy resigns, or their election is determined to be invalid, a by-election (French: élections legislatives partielles) is held instead.
For the Fourteenth Legislature (2012 - 2017), the youngest Deputy-Substitute in France was Nicolas Brien, born in 1989, who was elected in Allier's 2nd constituency.