Since 
it does not import in-scope schema definitions from the imported modules, the 
importing 
module should not see what schema definitions are there in the 
imported modules.Since they
could not see each other, then how could they 
decide whether their ISSD is "equivalent"? 
and it seems also meaningless to 
compare their ISSD because they will not disturb each 
other. 
 
This may be why the spec doesn't require an 
implementation to check that the two ISSDs are compatible, but merely says that 
the results are unpredictable if they are 
not. 

and


  If a module could see the imported module's 
  schema definition, then why spec. still
force the importing module 
  explicitly import necessary schema definition, since these types 
will 
  surely be imported from other module's ISSD?

XSLT took the 
  decision that all imported schema definitions would be available in all 
  modules. However, that makes it harder to do separate compilation of 
  modules. I think the rule in XQuery that each module must import all 
  the schema definitions that it needs is there in the belief that this will 
  make separate compilation of modules easier.
   
  Michael Kay
  http://www.saxonica.com/