LIS 2670 week 6 XML and Markup Languages muddist point

1, what dose the sentence " 80% of SGML' s power at 20% of its complexity." mean?

2, What is the difference between the URI in an XML namespace and the URL?

3, Which is more popular, DTD or XML schema? Speaking of digital objects, which of two do we use?

4, Why the defaults of minOccures and maxOccurres are both 1 in PPt45? I thought the default of minOccures is 0.