What is the "list-separator String appropriate for the host environment’s current locale" in JavaScript (ECMAScript)

Question

While reading the ECMAScript 2015 Language Specification, I came across the following step in the Array.prototype.toLocaleString method:

5. Let separator be the String value for the list-separator String appropriate for the host environment’s current locale (this is derived in an implementation-defined way).

My question is, how do we know what is the appropriate list-separator String for the host environment's current locale? Is there any standard for this? Can it be just any string (e.g. "asdfg")?