UEFA has been running a "Youth League" tournament in parallel with its Champions League for four years now. In the last two seasons, the competition has been expanded from 32 teams to 64, and it is in those two years that RB Salzburg has gained entry to the tournament.
Last season, Salzburg's U-19 squad was bounced by Roma in the final round of qualifying for the knockout stages for teams entering via the "Domestic Champions path" (the other route is the "Champions League path" - in which the youth teams of all 32 UEFA Champions League clubs play an identical group stage to that of their first teams).