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The current definition of "stylesheet level" is: [Definition: A stylesheet level is a collection of stylesheet modules connected using xsl:include declarations: specifically, two stylesheet modules A and B are part of the same stylesheet level if one of them includes the other by means of an xsl:include declaration, or if there is a third stylesheet module C that is in the same stylesheet level as both A and B.] This does not reflect what happens with used packages. I think these are on the same stylesheet level (if P uses Q and Q includes R then declarations in P, Q and R are all in the same stylesheet level, even though Q and R may have components that are invisible to P). Furthermore, I think it would be clarifying if we say something like "Note: if A imports through xsl:import B, then A is not on the same stylesheet level as B.". I'm not sure whether the definition itself should be updated, but it may help to clarify the relation between include/import/use-package.
We should probably clarify that a "level" is a subset of a package. There's a containment hierarchy: a module is part of a level and a level is part of a package. Two modules in different packages therefore can't be in the same level. That's implicit in the current definition but it could be made more explicit.
I have added the following note in the hope that this removes any possible confusion: <note diff="add" at="T-bug29666"><p>A stylesheet level thus groups the declarations in a package by import precedence: two declarations within a package are in the same stylesheet level if and only if they have the same import precedence.</p></note>
The WG decided to close the bug accepting the added note as sufficient clarification (but not without considerable discussion straying into the dangerous territory of the identity of stylesheet modules.)