2.1.567 Part 1 Section 18.2.15, functionGroups (Function Groups)
a. The standard does not limit the value of @builtInGroupCount.
Office requires that the max value be 255.
b. The standard states that the maxOccurs value for the sequence containing the child functionGroup element is unbounded.
Excel limits the occurrences of the functionGroup element to 255.
c. The standard states that the values of the builtInGroupCount attribute are defined by the XML Schema unsignedInt datatype.
Excel restricts the sum of the functionGroup elements plus the value of the builtInGroupCount attribute to be at most 255.