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Subject: Customization limitations

Dear UBL TC,

This is a comment to point out a weakness in reliance on customization
to implement UBL which might need to be noted in future documents
about customization and implementation of UBL (perhaps if any
revisions are made as a result of the PAS submission).

This comment also incidentally supports the decision to exclude a
formal calculation model from the UBL semantics so far.

There seems to be a serious limitation in the subset concept typically
applied to customizations, not only of UBL documents but perhaps more
generally too, in that in order to define the semantics of a subset,
there may be unintended variation introduced between the semantics of
the subset and the semantics of the superset. When this semantics
variation involves the calculation model (such as when a subset is
made of a customization which includes a calculation model), I think
it is likely to have adverse side-effects.

For example, in a document there may be an entity to apply a rounding
value to amount totals; if the subset does not include this entity but
the subset includes a calculation model, it is possible that the
calculation model will exclude the rounding value. This factor may be
irrelevant if the subset does not allow inclusion of non-subset
entities outside its own syntax in documents. However, if the subset
does allow a document to include the rounding value then its semantics
and in particular its calculation model will need to handle this as an
issue. In such a case, I think UBL needs to warn implementers, the
designers of the semantic subset need to be aware and make it clear
how such a document is to be handled.

The same applies, I think, to the case of multiple subsets where
interoperability is sought between them.

I suggest this be noted in future UBL documents and perhaps some other
means be used to warn UBL implementers.

Best regards
Stephen D Green

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