Hi Stefano,
Ad Param value will be used if the ad is being shown on the Google Search Network. If it is being shown on the Google Display Network, the default parameter will not be replaced with the AdParam value.
Ad Params can be used on description, headlinePart1 and headlinePart2 fields of an Expanded Text ad and description1, description2 and headline of a standard text ad. This
guide provides more information on using Ad parameters.
Regards,
Shwetha, AdWords API Team.