The Core Collections have a few title additions each month, as well as the occasional title update or retirement. The Principal Admin of each account should be receiving a monthly Content Update email with links to the updates for your account's Core Collection. If you are not the Principal Admin of your account, but would like to receive this email, just submit a support request and ask to be added to our monthly Content Update email list. You can also opt-in here.
We also host this information on the Help Center; just find your Core Collection on this Content Updates page. Most of our users subscribe to the Academic Core product, which was formerly known as Credo General Reference Premium. Some institutions will instead have the packages Student Core, Public Core, or Complete Core. You can get a definitive list of your subscribed collections within your Admin account (log-in required), or submit a support request and let us know you'd like to know the name of your collection.