Registrations can be done by: https://www.atletiek.nu/wedstrijd/inschrijven/35025/
For the moment only the athletes with results in 2019 or 2020 better than in table below are allowed to participate.
1 coach for 1 athlete, they can register: Click this LINK
The federation got a clear answer on the question, who's "ELITE".
U20 means year of birth 2002 or younger, U23 means year of birth 2001, 2000 and 1999...