Example of it from standard:
OnOpenPage trigger calls this function to set the variable value assigned to fields
local procedure SetDocNoVisible()
var
DocumentNoVisibility: Codeunit DocumentNoVisibility;
DocType: Option Quote,"Order",Invoice,"Credit Memo","Blanket Order","Return Order",Reminder,FinChMemo;
begin
DocNoVisible := DocumentNoVisibility.SalesDocumentNoIsVisible(DocType::Order, "No.");
end;