The main feature of 21.1 is Sharing Portal – a separate SharePoint Online Team site that can be created from Contentworker and associated with a matter. This site is used to store documents shared as secured links or checked out from Contentworker to Office 365 for collaboration. But there is no limitation on just using it for documents originating from Contentworker.
Apart from the above functionality, 21.1 also contains a feature that allows to add a Subject Filing Tag and send emails with fewer clicks, Tree View improvements and other minor fixes.
Please download release notes here for more information about new features and fixed issues.