Under the old CRS (before 2000) it used to be like this (not sure about the current NRS system)
A train is split into segments :
Kings Cross to Peterboro / Peterboro to York / York to Newcastle / Newcastle to Edinburgh.
Seat 01 in coach D can be reserved upto 4 times - 1 journey per segment - so it could be reserved Kings Cross to Newark (across 2 segments) and Darlington to Edinburgh but not from Newark to York as reservations already exist for that segment.
A train would have a quote of a particular ticket type. Quota's were identified by two letters - so ST may be the cheapest Advance, SW may be the next lowest, SX the next and so on.
The train would be set a limit, for example ST=20. You could have a small group of 20 people travelling on ST quota between Kings Cross and York and then another small group of 20 travelling on the same quota between York and Edinburgh. Therefore you have 40 ST quota tickets - but only 20 at once. If 20 people purchased an ST quota between Kings Cross and Darlington - they wouldn't be available again until Newcastle.
It is also possible to have barred journeys. You could stop people with the ST quota from travelling to Newark (let's say that there is a special event taking place in Newark and you don't want to sell cheap tickets there) or you could prevent the ST quota from a particular station (let's assume Newark again)
It was also possible to limit the amount sold for a particular journey so you could say KGX-YRK (Kings Cross to York) ST=5. In effect this would mean that out of 20 only 5 could be sold for a journey to York - leaving the other 15 to be sold for other journeys.
As in the above scenario you could limit just 5 tickets to York or 5 tickets for York and stations south thereof.
If I also remember rightly you could limit coaches as well. You could put all the St quotas in Coach B - furthest away from the buffet. This was always used with Standard Plus which allowed holders of full fare anytime tickets access to the standard plus coach which was the one closest to the buffet.
YOu could specify the order that coaches were reserved in - for example B,D,F,C,E or be really clever and say full fare tickets are reserved in sequence but advance tickets may go D,F,E,B,C.
There was also a logic into what seats were offered. Some seats are classed as being together (for groups 3,-8 people). If you are the first person to reserve on that train it would always offer B03A (unless it was overidden by a speacial request such as an aisle seat). The next person would alwats get B05A.
Hope this helps.