Second-string and 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 some cases you can use "Second class" instead an adjective phrase "Second-string".

Second class noun - Not the highest quality in a classification.

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