The notifications you got were for the U21 coach to "flag" your players (whom are 18ish) as being potential players for the u21 team. Its so they can then reference the player later to follow how they are trained and such to see if they will be viable options come the age 21 season (sometimes younger for smaller countries). For additional questions ask Nick at
https://www.buzzerbeater.com/team/28545/overview.aspx