Simplified, TOPS is a computerisd system whereby every loco, or other vehicle,is tracked . In that way they can, for example, keep tabs on when a certain vehicle requires overhaul, that sort of thing. To make it work all locos were renumbered - the first 2 numbers are the class, the third the subclass and the reaining 2 are the individual locos. This was introduced in 1971
In the 1980s the entire system was split into business units e.g NSE, InterCity and all costs incurred by each units were allocated to that sector. Freight was treated the same way. Each sector had its own locos, rolling stock and I believe even track, and if another sector wanted to use something 'owned' by an other they had to pay for it. e.g. the loco allocated for a prticular InterCity train fails - you have to borrow a Railfreight Coal to run the service. You have to pay a hire charge for the standin loco.
There's a lot more to it than what I've just put but I haven't the time, or knowledge, to go any further.
Hope this helps