What is meant as by an external source in JavaScript?

The ECMAScript language specification at section 4.4.2 which defines implementation-defined as:

• An implementation-defined facility is defined in whole or in part by an external source to this specification

Now what exactly is meant here by an 'external source'? Would an external source be considered some random JavaScript code put inside of some JavaScript features? Let's see with an example:

function func(){
     var a = 10;
     var k = "variable";
     var obj = {};
}

Would the external source in this case be considered the declared name, as well as the variables defined within the function?

Or if this isn't the case then could you explain what is meant by the above definition and what would be considered an external source?

Solution 1:

No, implementation-defined refers to the implementation of the JavaScript engine and native functions provided by the host application that uses the engine. It doesn't mean JavaScript source code executed by the engine.

The term "external source" refers to documents such as the Node.js API specification or the HTML specification and related web platform standards.