Auxiliary and Second string

Auxiliary adjective - Available to supply something extra when needed.

Second string and auxiliary are semantically related. In some cases you can replace phrase "Second string" with "Auxiliary", this terms are similar.

Second string noun - A squad of players that are available either individually or as a team to relieve or replace the players who started the game.

Auxiliary and second string are semantically related. Sometimes you can use "Auxiliary" instead an adjective phrase "Second string".