A URL is a 'universal resource locator': it refers to a web site, e.g. pleiades.stoa.org. A URI is a 'universal resource identifier': it refers to a specific item sheltered under the URL. So, in the pleiades scheme, 579885 is a URI that refers to Athens. And the link (URL + URI): pleiades.stoa.org/places/579885 is the pointer to 'Athens' in their database and which will be returned by Pleiades. This is a very friendly form and easy for the user to remember.
- Elliott  45-6.
- A guide to rewrite rules is here and a handy cheat sheet for regular expressions (RAs) is here.