SRS - Standard Routing Scheme. Eurocontrol decide which routes will be taken through individual countries airspace when flying between two city pairs. Heathrow to Paris has a specific route (at low level) through Seaford and Dieppe, which if not filed will either result in a flight plan rejection or a serious delay on the route when it actually comes to two hours before IOBT. They introduce the SRS routings regularly and update current ones fairly often - usually just as Ops have worked out the correct routing!!
There are lists of routings, and I mean lists... which need a person full time to ork them out and apply them. So there you go a brief introduction to SRS!