I'm on it this year and the selection criteria, at least in my county for it is "referees that the county believes are likely to be officiating at Level 4 within the next 5 years"
Its a little like the xFactor - we've got the following categories:
L7 - Under 25
L7 Over 25
L6
L5
Those doing 5-4 this year.
The first thing that will get you noticed will be a promotion application starting in March as thats the first step on the path. I believe the L7's and 6's selected are all on a current promotion season. The expectation of us L5's that are not is that we will be applying for 5-4 starting in March 2019.