When you choose to create a new message definition file from an existing WSDL file, you are presented with the following panels:
The check box on the left of each row is used to define whether you want the process to overwrite an existing file of the same name in the specified location. If the box has a tick in it and is grayed, then no file of that name exists in the specified location and the file will be generated.
If a file exists in the specified location with that filename, the check box will not be selected and should you proceed, these files will not be generated unless you select the check box. The existing files will then be overwritten.
If you change the filename, the state of the check box depends on whether a file with that name exists in the specified location. For example: