2.1.188 Part 4 Section 22.214.171.124, customXml (Row-Level Custom XML Element)
a. The standard states that the uri attribute can contain any string value.
Word will not load a file if the URI used in the uri attribute is not also contained within an attachedSchema element.
This note applies to the following products: 2007, 2007 SP1, 2007 SP2.
b. The standard states that the element attribute can contain any string value.
Word only allows values conforming to the XML specification; a string conforming to the definition of xsd:NCName with a length of at most 255 characters.