Second-class and Second string

Second-class adjective - Of little or less value or merit.

Second string and second-class are semantically related. In some cases you can replace phrase "Second string" with "Second-class", this phrases 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.

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