docs: add note

Author: kgryte

spec/draft/API_specification/indexing.rst

@@ -182,6 +182,9 @@ Multi-dimensional arrays must extend the concept of single-axis indexing to mult
 Integer Array Indexing
 ----------------------
 
+.. note::
+  Integer array indexing, as described in this specification, is a reduced subset of "vectorized indexing" semantics, as implemented in libraries such as NumPy. In vectorized indexing, integers and integer arrays are broadcasted to integer arrays having a common shape before being "zipped" together to form a list of index coordinates. This form of indexing diverges from the multi-axis indexing semantics described above (see :ref:`indexing-multi-axis`) where each element of an indexing tuple comprised of integers and slices independently indexes a particular axis. This latter form of indexing is commonly referred to as "orthogonal indexing" and is the default form of indexing outside of Python in languages such as Julia and MATLAB.
+
 An array must support indexing by an indexing tuple which contains only integers and integer arrays according to the following rules. Let ``A`` be an ``N``-dimensional array with shape ``S1``. Let ``T`` be a tuple ``(t1, t2, ..., tN)`` having length ``N``. Let ``tk`` be an individual element of ``T``.
 
 - Providing a zero-dimensional integer array ``tk`` containing an integer index must be equivalent to providing an integer index having the value ``tk[()]``. Conversely, each integer index ``tk`` must be equivalent to a zero-dimensional integer array containing the same value and be treated as such in all contexts, including shape inference and broadcasting. Accordingly, if ``T`` consists of only integers and zero-dimensional integer arrays, the result must be equivalent to indexing multiple axes using integer indices. For example, if ``A`` is a two-dimensional array, ``T`` is the tuple ``(i, J)``, ``i`` is a valid integer index, and ``J`` is a zero-dimensional array containing a valid integer index ``j``, the result of ``A[T]`` must be equivalent to ``A[(i,j)]`` (see :ref:`indexing-multi-axis`).