In adlibweb.xml you may specify a custom XSLT stylesheet which the WebAPI must always use to transform the returned XML result from searches in the current database. You need wwwopac.ashx version 3.7.15343.1 or higher if you'd like to use such a custom XSLT stylesheet only to add one or more (compound) fields to the resulting XML while maintaining the Axiell AdlibXML format (this is required) and you wish to output it as JSON (using the output=json API command). Prior to this wwwopac.ashx version, output to JSON disregarded any custom stylesheet you had set. You can use this functionality if you want to output to JSON and you need some compound field data, like an extra field containing the ISBD presentation of a publication, to be part of that output before it is processed any further.