The |oui filter [1] (since SPIP 2.0) returns either a space or nothing at all.
It is equivalent to |?{' ',''} or |?{' '}, and is used to return a non-empty content (a space) to indicate that the optional parts of the tag should be displayed.
Examples:
[(#TITRE|strlen|>{30}|oui) This title is a long one
[(#EMAIL|oui) #FORMULAIRE_CONTACT ]
See also the |non filter.