Hello, Irish Girl, and welcome to the Grammar Exchange.
Example A: The company, which is a subdivision of ACME, posted earnings today.
Example B: The company, a subdivision of ACME, posted earnings today.
Both sentences are correct. In (A), you have a relative clause after "the company," while in (B) you have an apposition.
When the verb in the relative clause is be, it may be a good decision to use an apposition (which is does not add any information if compared with the apposition). When the verb is not merely a copula, the relative (or an abridged relative) will be needed:
- The company, which was recently purchased by ACME, posted earnings today.
- The company, recently purchased by ACME, posted earnings.