In the interests of efficiency, the HTX linker is designed to generate references to locally-installed documentation whenever possible. If a referenced document cannot be found locally, however, it will instead generate a reference to a remote document server -- which should have a complete set of documentation available. In this way, links to documents that are not available locally should not fail, and the reader should always see a complete set of documentation.
The linker regards a document as remote if it is referenced by one of the documents it is linking, but it cannot find it either in the document libraries being linked or on the HTX_PATH search path.
Note that the HTX linker will not look up a document on a remote machine. All it does is to insert the remote server reference. References to invalid labels in remote documents (or, indeed, to non-existent documents) cannot therefore be detected at link time and diagnosing this type of error is the responsibility of the server itself.