Comparing snapshot revisions or versions is controlled by the presence of the ISPFAuthPublishableDocumentVersion interface definition. In the delivered model, the interface is a required realizes relationship so the functionality is available by default.
The publishable document compare uses the existing DocVersionCompare method. This method was previously only available on published documents, but has now been made available to publishable documents (snapshot documents) through the ISPFAuthPublishableDocumentVersion interface definition.