The|xou filter [1] (since SPIP 2.0) is used to check the existence of only one single element out of 2 possibles.
Wikipedia offers a complete definition of the Exclusive OR.
Examples:[(#CHAPO|xou{#PS}) Either there is a stand-first, or a post-scriptum, but never both][(#CHAPO|xou{#PS}|non) There can be neither stand-first nor post-scriptum, or there can be both at the same time]