How to write an external binding file for xjc?

The documentation of JAXB xjc says:

-b Specify one or more external binding files to process. (Each binding file must have it's own "-b" switch.) The syntax of the external binding files is extremely flexible. You may have a single binding file that contains customizations for multiple schemas or you can break the customizations into multiple bindings files:

   xjc schema1.xsd schema2.xsd schema3.xsd -b bindings123.xjb

   xjc schema1.xsd schema2.xsd schema3.xsd 
         -b bindings1.xjb -b bindings2.xjb -b bindings3.xjb 

In addition, the ordering of the schema files and binding files on the command line does not matter.

But where do I find a documentation of this "external binding file"?