Reserves and Second string

Reserves noun - Civilians trained as soldiers but not part of the regular army.

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

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.

Reserves and second string are semantically related in group of substitute players topic. You can use "Reserves" instead a noun phrase "Second string".