Why doctype is used in XML?

Related questions

1. What is SGML and DTD?
2. Does html5 support offline storage?
3. Is html5 compatible with older browsers?
4. How do you know which HTML version I am using?
5. Where is my website HTML?
6. Does html5 need doctype?
7. What is doctype >? Is it necessary to use in html5?
8. What DTD means?
9. Is doctype case sensitive?
10. What is doctype in XML?

Tags

1. #declaration
2. #schema
3. #referenced
4. #named
5. #doctype
6. #document
7. #reference
8. #retrieve
9. #validate
10. #contains
11. #actual
12. #which
13. #external
14. #detail
15. #systemidentifier
16. #element