The sub-documents are: frames, iframes and embedded objects (flash tubes in general); temporary redirects (often used to place cookies or redirect to a page, depending on the language preferences of the user); versions of the same page in different languages obtained with Content negotiation.
The indexing of sub-documents is controlled by two commands: the SubDocLevel command sets the maximum nesting level of a sub-document to be indexed. The default value is 0, which prohibits the sub-document indexing. The SubDocCnt command sets the maximum number of sub-documents to be indexed at all nesting levels (this command is mainly to prevent endless cycles of pages nested into each others). The default value is 5.