If the tag’s output is not a numeric type, it will be considered as being 0 (zero) and the filter will simply return 0 (zero):
[(#TEXTE|mult{18})]
will return "0".
It is possible to assign a negative number as the argument to the filter:
[(#TOTAL_BOUCLE|mult{-5.2})]
Warning: if xx is a floating point number, it must be written with a period as per the English method as opposed to being written with a comma as some European languages do:
[(#TOTAL_BOUCLE|mult{5.2})]