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In a nutshell, this boils down to whether relative references are allowed, and whether support for them is required. I can find nothing in the LC2 draft answering this question definitively, although I suspect the working group's intent was that the content was a URI reference (i.e. relative URIs are allowed, and support for them is required). The working group might also consider making explicit the contextual expectation that the content of documentURI is usually a URL (a subset of all URIs).
Discussed on telecon of 2008-11-20. Change 5.2.2 from: SML-IF specifies one way that MAY be used to provide the location of the referenced document, the documentURI element. To add the following to the sentence above: It is a consequence of documentURI's schema definition that it contains a URI reference, i.e that it may be an absolute URI or relative reference. When it is a relative reference, the [base URI] property SHOULD be used to transform it to an absolute URI, as stated in [5.3.2 Base URIs].
Fixed per comment #1.