A different use of square brackets from list identification is that of addressing a key in a Dictionary. Doing so causes the value of the address key to be returned. So:
$MyDictionary = {car:hatchback;ship:ketch;plane:cargo};
$MyValue = $MyDictionary[ship]; gives 'ketch'
In the past, string quoting rules are interpreted as meaning key terms with quoted but this is no longer best practice—see the article on {} Dictionary definition.