ECP internal and external url

I am trying to understand the ecp virtual directory external URL. If I don't set it and leave it empty, does it mean outside my network (from internet) that it can't be accessed? Currently i have both internal and external set up as 'mail.mycompany.com'. From both Internet and intranet, it can be accessed using https://mail.mycompany.com. If I remove external url from Exch Admin Center, does it mean it can't be accessed from Internet? How does it know the connection is from Internet or from intranet?

THanks.

Alan