The WMS specification allows the use of vendor parameters in requests. Vendor parameters are non-standard (that is; the parameter is not part of the OGC specification) request parameters that have been implemented by a vendor to provide enhanced capabilities. While vendor parameters are useful to provide enhanced capabilities, care must be taken when using them in your own client-side implementations. If you are making a generic client application that can target multiple WMS servers, then it is quite likely that they will be from a range of vendors. If you use a vendor-specific request parameter to a server that is not aware of it, you might see errors occurring. However, if you know for sure that you are targeting a GeoServer WMS, then there are a number of vendor-specific request parameters available.
Tip
A complete list of the vendor-specific WMS parameters that GeoServer supports is available at http://docs.geoserver.org/2.4.x/en/user/services/wms/vendor...