array element
The elements of an array are variables, and are used in expressions just like normal variables, as in the following examples:

FUNCTION Demo (w, x, y, z$)
  SHARED a%[]                 ' a%[] has SHARED scope
  SHARED k[]                  ' k[] has SHARED scope
  STATIC j$[]                 ' j$[] has STATIC scope

  IFZ j$[] THEN DIM j$[63]   ' DIM j$[] on first entry
  a%[w] = x                   ' wth element of a%[] = x
  k[z] = x * a%[w] * a%[y]   ' use arrays in an expression
  j$[x] = z$                  ' put a string in j$[x]

array implementation
One-dimensional arrays are implemented as a contiguous set of elements, much like other languages.  Multi-dimensional arrays, however, are implemented as multi-level trees of one-dimensional arrays.  As long as they are created and used in the conventional manner, this implementation difference is invisible. But tree-structure arrays have enormous advantages.

tree structure
Multi-dimensional arrays are arrays of array addresses, except for the lowest dimension which contains data.  This permits the construction of irregular arrays, for which there are supporting features.

regular array
There's no difference between conventional vs tree structure, or regular vs irregular in one dimensional arrays.  And multi-dimensional arrays behave just like conventional arrays as long as the features provided to create and manipulate irregular arrays are not employed.

irregular array
The distinction between regular and irregular arrays applies only to multi-dimensional arrays.  Furthermore, irregular arrays need not be built or manipulated until and unless their special properties are needed.  Many programmers may never need them.   On the other hand, system programs, and other sophisticated applications are often vastly more efficient and easier to implement with irregular arrays.

Regular arrays have the same number of elements in every instance of a given dimension.   In an array created by DIM a[3,7], the upper bound of the low dimension is 7 for all three upper dimension values (a[0,7] , a[1,7] , a[2,7], a[3,7]).  Regular arrays can be thought of as rectangular.

Irregular arrays don't need to have the same number of elements in every instance of a given dimension.  For example, the upper bounds of the low dimension of a similar irregular array could be b[0,3] , b[1,*] , b[2,5] , b[3,2] (where "*" means no elements are allocated in this subdimension).  Therefore, irregular arrays cannot be thought of as rectangular, but can be thought of as ragged arrays.

nodes and data
Arrays contain two kinds of contents, nodes and data. Elements in the lowest dimension are data. Elements in higher dimensions are nodes, meaning addresses of sub-arrays.

Consider a[], created by DIM a[3,7] .  Five one-dimensional arrays are created.   The first array corresponds to the higher dimension, a[3, ], and contains elements 0 to 3.  These elements are nodes, addresses of four arrays that contain data elements 0 to 7.