Try the following in iex:
iex> ['cat' | 'dog']
['cat', 100, 111, 103]
When using the [head | tail] operator, head is a single element whereas tail is the list of remaining elements. So, when we say ['ab' | 'cd'], the first element of the list becomes the sublist 'ab'. This is the same as saying:
[[99, 97, 116], 100, 111, 103]