Some work item customization questions

1. I was going through the help and and want to know about the need for defining mapping between change management type and work item type

http://publib.boulder.ibm.com/infocenter/clmhelp/v3r0m1/index.jsp?topic=%2Fcom.ibm.team.workitem.doc%2Ftopics%2Ft_defining_cmtype_witype_mapping.html
----------------------------------

2. In the "workflows" section why there should be mapping between "state groups" and "OSLC groups"? What is the significance?
----------------------------------

3. "Enumerations" section - To make an enumeration attribute a required field, define an Unassigned literal and select this literal in the Unassigned Literal field. I didn't understand this part.

http://publib.boulder.ibm.com/infocenter/clmhelp/v3r0m1/index.jsp?topic=%2Fcom.ibm.team.workitem.doc%2Ftopics%2Ft_defining_required_properties.html
----------------------------------

Thanks!