The RequestEncoding property specifies the assumed encoding of each incoming HTTP request, including posted data and query-string data. If the request comes with a request header containing a ContentType field, the AcceptCharset field of the request header overrides the RequestEncoding of the configuration.
The default encoding is UTF-8, specified in the globalization section of the Machine.config file that was created when the .NET Framework was installed. If the RequestEncoding property is not specified in the Machine.config or Web.config file, encoding defaults to the Regional Options locale setting on the computer. For single-server applications, the RequestEncoding and ResponseEncoding properties should be the same. When using multiple server applications where the default server encodings are different, you can use local Web.config files to vary the RequestEncoding and ResponseEncoding properties.