What is the PMC Open Access subset? Isn't everything in PMC open access?

The majority of the articles in the PubMed Central^® PMC database are subject to traditional copyright restrictions. You can access them without charge, but they are not Open Access articles in the specialized sense of that term.
The PMC Open Access Subset contains articles that are still protected by copyright, but are made available under a Creative Commons or similar license that generally allows more liberal redistribution and reuse than a traditional copyrighted work. See the PMC Open Access Subset page for more information on how you may retrieve and use these articles.