An array with a single atom is referred to as a singleton. All singletons with the same atom display the same way. However, the fact that they have different ranks affects how verbs act on them. This can be a pitfall for beginners. It is important to remember that if it displays like an atom, but does not behave like one, then check its rank.
atom =. 5 list =. 1 $ 5 atom 5 list NB. list looks like atom 5 atom + 23 23 23 28 28 28 list + 23 23 23 NB. but does not behave like atom �length error � list +23 23 23 # $ atom NB. rank of atom is 0 0 # $ list NB. rank of list is 1 1