Junior and Second string

Junior noun - One who is of lower rank and typically under the authority of another.

Second string and junior are semantically related. In some cases you can use "Second string" instead a noun "Junior".

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.

Junior and second string are semantically related. You can use "Junior" instead the phrase "Second string" as a noun or an adjective.