1.02 These rankings (Annexe Ia), drawn up in accordance with the stipulations of
Annexe II, points 5 to 10, determine the number of representatives per
association, as well as the stage at which they join the competition.
Associations are represented on the following basis:
a) Two representatives: winners of the domestic cup and the club which
finishes the top domestic league championship immediately below the
club(s) which qualify for the UEFA Champions League.
b) Three representatives: winners of the domestic cup and the two clubs
which finish the top league domestic championship immediately below
the club(s) which qualify for the UEFA Champions League.
c) Four representatives: winners of the domestic cup and the three clubs
which finish the top domestic league championship immediately below
the club(s) which qualify for the UEFA Champions League.
Taken from the book of regulations of the UEFA cup.
Read it here