Res judicata means "the thing has been decided" and derives from the principle "interest reipublicae ut sit finis litium" (it is for the good of the commonwealth that there be a terminus to litigation).
Canadian doctrinal and judicial writing on functus officio is sparse,[2] even though the rule itself derives from a 19th century case of the English Court of Appeal.
[3] In the case of Chandler v Alberta Association of Architects, Sopinka J. summarized in relation to the principle of functus officio: "The general rule (is) that a final decision of a court cannot be reopened ... the rule applied only after the formal judgment had been drawn up, issued and entered, and was subject to two exceptions: where there had been a slip in drawing it up, and where there was an error in expressing the manifest intention of the court."
[5] However, the underlying rationale for the doctrine is clearly more fundamental: for the due and proper administration of justice, there must be finality to a proceeding to ensure procedural fairness and the integrity of the judicial system.
It is based, however, on the policy ground which favours finality of proceedings rather than the rule which was developed with respect to formal judgments of a court whose decision was subject to a full appeal.
[7] This concern for finality is evident in the definition of functus officio:[8][translation] Qualifies a court or tribunal, a public body or an official that is no longer seized of a matter because it or he or she has discharged the office.
[11] Whether in its common law or statutory form, the doctrine of functus officio provides that only in strictly limited circumstances can a court revisit an order or judgment.