2.1.1644 Part 1 Section 22.1.2.3, aln (Alignment)

a.   The standard states that aln is not used when applied to a box which is not an operator emulator.

Office does not allow aln to be applied to a box which is not an operator emulator.

b.   The standard says aln applies only to the box function even though the schema allows it as a child of math runs.

In Word, the aln element applies to the box function and to math runs.