Simple question, I hope.
Is a TfL one-day travelcard a 'zonal ticket' for the purpose of splitting thus allowing me to combine it with a point-to point ticket for the rest of my journey on a train that doesn't stop at the changeover point?
Depends how you do it. The travelcard is zonal, but the splitting regs require both tickets to be zonal, which a point-to-point isn't. However, if you buy a boundary zone X ticket to join up with the travelcard then the train doesn't have to stop.
Ticket machines don't give option to buy "edge zone X" tickets, sow how are you meant to book a through journey (eg from a monthly Travelcard )
Boundary Zone tickets will be "Any Permitted" so may offer more flexibility than a TOC specific point to point ticket.
Southern machines sell boundary zone tickets and allow you to buy tickets starting from somewhere else, so pretty versatile.
OK rephrase my assertion. In (poss distantish) past they didn't do it and I have always been too much of a hurry to recheck. Or I feel hassled by the queue behind.