Second-string and Under

Second-string adjective - Being a replacement or substitute for a regular member of a team.

Under and second-string are semantically related. in importance topic. In some cases you can use "Under" instead an adjective phrase "Second-string", when it comes to topics like minor, lesser.

Under adjective - Located below or beneath something else.

Second-string and under are semantically related. Sometimes you can use "Second-string" instead an adjective "Under".