Maintaining document links
The fundamental weakness of system identifiers is that they specify the exact, known location of a resource, so they fail to work if the resource is moved, or if the original resource is not accessible, but a copy elsewhere is to be used instead. While relative URL references (see Chapter 30) can be helpful when linked resources are moved together to a new location (and directory names involved in the bundle are not changed), they do nothing to avoid the other problems discussed here.
Avoiding this problem requires the use of entity management techniques, and a different kind of identifier.
An entity manager uses public identifiers to locate external entities. Entries in catalogue files simplify the management of entities ...
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