This is an internal type used by the interpreter for processing expression evaluation results. It cannot be stored as an object property.
Although the List type does not really exist as a data type accessible from a script, the internal behavior of the interpreter appears as if it did. Especially when looking at the way argument lists are processed in new operator expressions.
ECMA 262 edition 2 - section - 11.2.4
ECMA 262 edition 2 - section - 8.8
ECMA 262 edition 3 - section - 8.8
ECMA 262 edition 3 - section - 11.2.4