If you write <xs:appinfo source="...">...
Then unless the source attribute is EXACTLY the right one, whatever is inside it will simply be ignored.
We need to warn if the source attribute is NOT what we expect and will be ignoring it.
Otherwise very wierd errors where say, a discriminator is simply not seen or processed can silently occur just because you have a typo in the source attribute of the appinfo.
This is a HUGE time waster.