2.1.550 Part 1 Section 17.18.78, ST_Shd (Shading Patterns)
a. The standard specifies an enumeration value "nil" with the same meaning as clear.
Word ignores value "nil" when used to specify properties of a paragraph or a run of text. Word treats the value "nil" as equivalent to not specifying shading when it is used to specify properties of a table, cell, or row.