iText Core includes about 500 functions which we license as a single SDK. This includes creation, merging, tagging, digital signatures, or any feature you would need when programmatically working with PDFs. iText Core add-ons provide robust functions required by smaller subsets of our customers.
You can refer to the iText Suite Features table to get an overview of which features are included in iText Core, and see the additional functionality provided by iText Core add-ons.