A key group is a set of metadata fields in a work item type that uniquely identify any given work item created for that work item type.
To add a metadata field to the key group, select its check box in the Key column.
You can also add metadata fields to the key group, as well as change the order of the fields in the group, through the Key Order dialog. Right-click anywhere in the metadata table and select Key Order from the shortcut menu.
All available fields that have not been added to the key group are listed in the Available Fields box, and those fields that already belong to the key group are listed in the Key Group Members box.
To move a field between the two boxes, click the field to select it, and then click the left or right arrow button to move the field from one box to the other.
To move a field up or down in the Key Group Members box, click the field to select it, and then click either the up or down arrow button until the field is in the appropriate position.
When you are done setting the key order for the work item type, click OK.