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?