Definition of "unreserved"

For the steam services, passengers with reserved seats are assembled behind the appropriate notice boards in the centre cab road, which is barred to motor traffic on summer Saturdays. Unreserved passengers line up behind other boards, which merely display destinations and not departure times.

1962 April, J. N. Faulkner, “Summer Saturday at Waterloo”, in Modern Railways, page 263

It cannot be right that someone who joins a train at 0800, having paid perhaps £200 for a single at the ticket office and who sits in an unreserved seat, can be removed from that seat at 1200 by someone who joins en route with a cheap Advance fare that he didn't even buy until 1100, and yet which came with a compulsory reservation. That turns our first passenger's seat from unreserved to reserved midway through the journey and is, to me, commercially immoral.

2020 July 15, Barry Doe, “Problems persist with Advance on the day”, in Rail, page 65