Publisher Policy Files and Side-by-Side Execution
A publisher policy file is an XML-based assembly, created by a component developer and released with a component upgrade, that sets policy on the version of the component that should be used. The file redirects assembly binding to use the assembly shipped with the publisher policy file. You can configure an application to ignore the policy file and use the version of a component that the application was built with.
Component developers should use caution when shipping a publisher policy file. The policy file can unintentionally affect other applications because it directs all applications that use a component to the new version of the component. By shipping publisher policy, you are stating that all applications that used the old component can use the new one.