The ordering properties of a set can depend on the data in the set, and not on use. I am attaching an example, with comments, that should make this clear. Note: The functions sameas, .pos, and .val will also work, if the set is not ordered.

* $ontext
* Here, the set 'big' is not ordered; this is because the
* elements in it have already appeared (in the sets even and odd),
* but appeared in the order 0, 2, 4, 1, 3, 5,
* not 0, 1, 2, 3, 4, 5
* Thus you will get an error message with the code below:
* 198   Set used in 'ord' or lag is not ordered.
*         Hint: Some of the elements of the set were used before this
*         was initialized and the order was different from the order used
*         in this set. Try to initialize the set earlier.

SETS
  even  / 0, 2, 4 /,
  odd   / 1, 3, 5 /,
  big   / 0 * 5 /;
* $offtext


$ontext
* Here, the set 'big' is ordered
* and the elements in even and odd occur in this same order,
* so they are ordered as well.
SETS
  big   / 0 * 5 /,
  even  / 0, 2, 4 /,
  odd   / 1, 3, 5 /;
$offtext


parameter oo(big);
oo(big) = ord(big);
display oo;