This is the usual way of referencing row and column vectors. But it can also apply to a fully two-dimensional matrix, in which case the array is regarded as one long column vector formed from the columns of the original matrix. If you try to use the value of an element outside of the matrix, it is an error.
Index exceeds matrix dimensions.
On the other hand, if you store a value in an element outside of the matrix, the size increases to accommodate the newcomer.