Query about context object

Hi Expert,

While reading about context object, I came to know that it is defined in ESR and it can be used in receiver determination of ID. So my question is how receiver determination step is able to identify that which context object contains which Xpath because the context object is defined in ERS not in ID...??

In other words how the context object value is fetched in ID which is defined in ESR not ID...??

Thanks in advance