Field Reference Names
An important objective of the work item type definition language and the work item query language is for definitions to be portable between Team Foundation Servers. It is also important for third-party integrations to be able to find and refer to specific fields. To this end, the work item type definition language includes the concept of a field reference name. Field reference names are globally unique in the same way that a namespace in the .NET Framework is globally unique.
For this reason, field reference names cannot be renamed. If, for example, you changed the field name "Title" to "Header," the field reference name of that field remains the same.
In keeping with the .NET namespace tradition, two namespaces are predefined: System and Microsoft. The System namespace includes all system fields that are mandatory for Team Foundation system functions. The Microsoft namespace defines all required fields for work item types defined by Microsoft. Customers and partners can create their own field namespaces for custom work item types.