> Institutions subscribing to ACM Open receive full access to the Premium version of the ACM Digital Library, providing their users with unrestricted access to over 800,000 ACM published research articles, the ACM Guide to Computing Literature (which indexes more than 6,500 3rd party publishers with direct links to the content), advanced tools, and exclusive features.

What does this mean? The 800,000 previously published articles will stay paywalled and only the new stuff will be open? Or will stuff be open to individuals while institutions have to keep paying? Or what?

So all articles will be open and free to read. The ACM Open subscription mainly includes publishing at a lower overall cost than the per-article rates, but also includes "AI-assisted search, bulk downloads, and citation management" and "article usage metrics, citation trends, and Altmetric tracking".