2.1.670 Part 1 Section 18.3.2.10, top10 (Top 10)
a. The standard does not mention any limits on the value of @val.
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In Office, if this element is on a pivottable, the @val must be greater than or equal to 0. Otherwise, it must be between 1 and 500 inclusive.
b. The standard does not specify a default value for the filterVal attribute.
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Excel uses a default value of 0 for this attribute.