In message , at 08:37:14 on Sat, 17 Mar
2018, Roland Perry remarked:

Much of the wriggling is because the Elizabeth Line (as discussed here
wrt roundels) is "neither fish nor flesh nor fowl" when it comes to
being a tube, an Overground or a National Rail line, having as it does
a station at Heathrow which both is and isn't in Z6 depending on the
ticket you buy.

I think I'll change that last bit to "or fare you are charged", because
until your day's trips are amalgamated under the capping system, all you
actually "buy" with smart ticketing is a trip through a gate.
--
Roland Perry