2.1.518 Part 1 Section 17.16.20, format (Text Box Form Field Formatting)

a.   The standard does not limit the length of the value of the val attribute.

Word restricts the value of this attribute to be at most 64 characters.

b.   The standard lists four possible values for format when type is equal to regular.

Word also uses the values "Single byte ..." and "Double byte ...".