Taking this back a step, if it was a season from Maidenhead to London Terminals, where would it be valid to? (I see maps that show valid routes to Kensington Olympia but I can't see how there are rail services to get there from Routes from Maidenhead via Slough)
Let's see:
Maidenhead has Reading and Slough as Routeing Points.
To determine the "appropriate" RP(s), we compare Standard Class Single fares from each to London with that from Maidenhead to London.
Thus:
Maidenhead - London Terminals Anytime Day Single is £9.80.
Reading - London Terminals Anytime Day Single is £13.60 rte Staines, £18.00 rte Any Permitted. Therefore Reading is
NOT an appropriate routeing point for a journey from Maidenhead to London.
(Stating the bleedin' obvious there. - Ed)
Slough - London Terminals Anytime Day Single is £7.40. Therefore Slough
IS an appropriate routeing point for a journey from Maidenhead to London Terminals.
Permitted routes from Slough to London Group can be found on Map RG, which permits Slough to Paddington direct.
Slough to Clapham Junction (via Kensington Olympia) is also on the map, but validity would be somewhat dubious as neither station is part of the London routeing group, nor the London Terminals ticketing group. (And I don't
THINK there are any direct services at the moment, or are Southern providing the token service at the moment?).
Going back to the OP's original question, here's what "The Manual" has to say on the subject of Day Travelcards:
ATOC said:
Day Travelcards - Available in both 'Anytime' and 'Off-Peak' versions for travel 'In-boundary' in selected zonal combinations and 'Out-boundary', where available, to London Zones 1-6 only. Day Travelcards are valid on the date shown on the ticket throughout the zones shown on the ticket, and for any journey wholly within the zones that starts before 0430 the following day. 'Out-boundary' Day Travelcards additionally include one return journey from the origin station to Boundary Zone 6.
Whilst the "Fares Finder"
does show a "route Windsor" option for a ticket from Boundary Zone 6 - Maidenhead (at a significant premium over the Any Permitted fares), no such option is given for Maidenhead - All Zones, so I would surmise that such a ticket would
ONLY be valid on services heading into Paddington.
Cheers,
Barry