An array is an ordered arrangement of elements (subscripted variables)  in
  up to 32 dimensions.  You can explicitly declare the data type and size of
  arrays in a DECLARE, COMMON, MAP, or DIM statement.

1  –  Virtual arrays

  Virtual arrays are arrays that are  stored  on  a  mass  storage  device
  rather than in main memory.  You use them just as you use normal arrays,
  but you must specify a different form  of  the  DIMENSION  statement  to
  allocate  them.   You  should  explicitly  initialize  all  variables in
  virtual arrays at the start of your program.

  Virtual arrays are supported to provide  compatibility  with  BASIC-PLUS
  and  BASIC-PLUS-2  and are not recommended for use in development of new
  BASIC programs.

2  –  Bounds

  When you create a nonvirtual array, BASIC allows  you  to  specify  both
  upper  and  lower  bound values.  If you specify both an upper and lower
  bound, the upper bound must be preceded by the keyword TO.  In addition,
  the  lower  bound must be less than or equal to the upper bound.  If you
  specify only upper bounds,  you  must  separate  them  with  commas  and
  enclose   them   in   parentheses.   Noninteger  bounds  and  noninteger
  subscripts are truncated to integer values.  Note  that  the  amount  of
  storage the system can allocate depends on available memory.  Therefore,
  very large arrays may cause an internal allocation error.

  Note that you can use the LBOUND and UBOUND  functions  to  return  both
  upper and lower bound values.