Paul Kelly
Verified Rep - BR Fares
If it is a train on which Southeastern sell their own advances from Kent to London, it will be shown in the timetable as reservable and you will get a "counted place reservation" detailing the train but not a seat when travelling on it with any advance ticket, whether it's only to London or further afield (i.e. as a connecting leg into a longer distance journey). But if it's not a train on which Southeastern sell their own advances, it may still be booked as a connecting leg on an advance for for which the main leg involves travel on another TOC (providing it complies with the time restrictions Southeastern impose on other TOCs' advances). In this case you won't get any reservation coupon (with old-style tickets) or the Southeastern leg won't be printed on the main coupon (with new-style tickets). I agree it is confusing.However, I am confused by the issues of being non-reservable versus booked train only. I understand that seat reservations are not available for many services, but in my experience I normally get a coupon stating the booked train time on it.
This link shows you all services from Sittingbourne to St Pancras today:
http://www.brtimes.com/#!board?stn=SIT&filt=SPX
The only ones on which you will get a reservation are those marked with a diamond symbol.