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 use "Second-string" instead an adjective phrase "Second-class".

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

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