XMI12 — Effect of xmi.import is unclear (medium)

The text just says that xmi.import 'identifies additional documents that are
needed to process the current document' but does not say what effect/benefit
the xmi.import has. For example, if one uses xmi.import to reference an
external file, does this make all the elements available in the same xmi.id
address space as the current document (hence it is an error if the imported
document has an xmi.id clash)? The example in A.3 still uses hrefs to refer
to an element (Department.xml#Instructor) in the imported file
Department.xml which implies that the xmi.import is completely redundant
since such an href can be used without the xmi.import being required.
Moreover the usage of the xmi.name and xmi.version attributes are not made
clear: is it a means of selecting a specific model from the referenced file?
(The example does not use version). What is the impact of referencing a
model compared to the other information that may be in the file? Can one
only import a model? Does the xmi.import fail if the file does not contain
the referred-to version of the Model?