Part 3, section 8.51 StructureField, defines the metadata for a field. It includes:

ArrayDimensions: This field specifies the maximum supported length of each dimension. If the maximum is unknown the value shall be 0.
The number of elements shall be equal to the value of the valueRank field.
This field shall be null if valueRank ≤ 0.
The maximum number of elements of an array transferred on the wire is 2147483647 (max Int32).

"The number of elements shall be equal to the value of the valueRank field." is incorrect. It should read "the number of dimensions shall be equal to the value of the ValueRank field."

Beyond that, it implies that for ValueRank > 0, this field is required.

In part 6, section 5.2.6, the example contains a table that shows how the different fields are encoded for Type1. field [1] is for the array of Y, and the table contains the entry ValueRank = 1, but the ArrayDimensions is not included. It should be mandatory.