The revision of these two sentences that uses a relative clause is "The orchestra that was seated on stage played a difficult score".
Relative clauses are clauses that start with a relative pronoun. In this case, the relative pronoun that has been used is "that" and the relative clause is "that was seated on stage". Furthermore, a relative clause is used to identify or define the noun that precedes it. In this case, the noun identified by the clause is "orchestra". Therefore, the relative clause "that was seated on stage" is postmodifying the noun "orchestra".