The solution files tab allows you to add different file submissions with different verdicts. The main solution it is only used to generate the output for each test case.

If you go to the checker tab, then you will be able to add a checker. This file is intended to read the Main solution answer and the participant's answer to compare them. So, in this file (the checker) you should check whether the participant's solution is ok or is wrong by reading the answer and check if it is a valid solution, depending on your problem.

Take a look at the examples to see how to read the author and participant's answers:

Github examples

Compare a single integer from author's solution with participant's solution