key = "string"; 

Specifies a name that the program uses to refer to this specific tile. Possible value is a quoted string (no default). Within a particular dialog box, each key value must be unique. This string is case-sensitive: if you specify the key as BigTile, you cannot reference it as bigtile.

Because the value of a key is not visible to the user, its name can be whatever you choose (as long as it is unique to the dialog box). For the same reason, key attributes do not need to be translated for applications offered in multiple languages.