Defines the unique identifier. The unique identifier is an application-defined string that corresponds to an item. The string can optionally consist of two parts: a unique existence of the item and any instance information. The first and second parts of the ID are separated by the at symbol (@) and cannot contain the at symbol. To determine whether two items are the same item, your application should compare the first part of the IDs (up to the at symbol) for both items.
<uid> <restriction base="string" <pattern value="[^@]+(@*)?"/>