Why does the alpha specification call the variable-length array length "implicit"?

In Section 3.7.4.2 “Variable-length array types” the length field is described as “implicit”. Why is it implicit? It seems fairly explicit to me. Am I missing some subtlety?

I described it as implicit because it is never specified directly by the data type author. Instead, it is constructed automatically based on the explicit properties of the array (its capacity).