Broadly speaking, links are grouos of work on the roster that a specifc group of drivers will work. Cross country covers so much geography it’s not practical for all Birmingham drivers to sign all of it.
I don’t know the detail here, so what follows is fictional but used as an example.
EG:
Link 1 will be work the Class 170s from Birmingham to Leicester, plus cover Gloucester to Nottingham
Link 2 will work the 170s, but cover Birmingham - Cambridge, and Cardiff to Nottingham
Link 3 will work 22x, covering Sheffield to Bristol and Cardiff via Gloucester,
Link 4 will work 22x, covers same as 3 plus Manchester to Bham incl all diversionary routes, and Bham to Reading
etc
Drivers will automatically move up the links as people leave (usually through retirement). So if someone from the ‘top’ link (often the link with the most route / traction knowledge) retires, one driver eill step up from the next link, creating a gap, whcih twkes someone from the next link, creating a gap, and so on. And they will all need training on routes and/or traction at the same time before being sufficiently productive to work the roster. So one retirement can trigger a whole cascade of people who become unproductive, or at least less productive, for a while.
This is a gross simplification, but I hope it helps.