2.1.839 Part 4 Section 220.127.116.11, rcmt (Revision Cell Comment)
a. The standard specifies that @author is restricted to be of type ST_Xstring.
Office defines that the length of @author should be between 1 and 52 inclusive.
b. The standard places no restrictions on the relationship between the guid attribute and a revision record.
Office uses the guid attribute to associate a revision record with a comment in a worksheet.