At the moment there are five different popular process templates, three from Microsoft official, one on Codeplex (by some Microsoft guys afaik), and one from Conchango.  The fields defined in the workitemtypes found in these templates belongs to a set of namespaces.  The System namespace is a predefined Microsoft namespace, with special behavior.  The other ones are in reality free text.  However, since some effort has been placed on defining these namespaces, and some of these fields are used in several reports, in mappings to Microsoft Project etc., it is wise to adhere to these definitions. It will at least reduce the work you have to do if you're modifying them, or defining your own types.  It is also wise to keep the fields named as equally as possible between types, and even processes, because it also simplifies querying across types. See http://teamfoundation.blogspot.com/2008/05/work-item-customization-tidbits-part-1.html for a great tutorial on how-to-do-it.

At the moment there are five different popular process templates, three from Microsoft official, one on Codeplex (by some Microsoft guys afaik), and one from Conchango.  The fields defined in the workitemtypes found in these templates belongs to a set of namespaces.  The System namespace is a predefined Microsoft namespace, with special behavior.  The other ones are in reality free text.  However, since some effort has been placed on defining these namespaces, and some of these fields are used in several reports, in mappings to Microsoft Project etc., it is wise to adhere to these definitions. It will at least reduce the work you have to do if you're modifying them, or defining your own types.  It is also wise to keep the fields named as equally as possible between types, and even processes, because it also simplifies querying across types. See http://teamfoundation.blogspot.com/2008/05/work-item-customization-tidbits-part-1.html for a great tutorial on how-to-do-it.