2.1.829 Part 1 Section 18.14.1, cell (External Cell Data)
a. The standard does not restrict @t having value s, which implies v contains a reference to a shared string table.
In Office, @t shall not equal s.
b. The standard does not specify how the index for the vm attribute is used.
In Office, the vm attribute shall be a one-based index into the valueMetadata collection in the metadata part that specifies this cell’s value metadata.
c. For @r, the standard does not restrict cell reference to be within the row of the parent row element.
For @r, Office requires that cell reference be within the row of the parent row element.
d. The standard implies that the r attribute is optional.
Excel requires this attribute.