Above is a screenshot of the ACL manager in Beta 1. It's accessible by clicking on the three-dot menu next to the dataset entry under Storage in the new UI.
Path - indicates the path that's ACL is being displayed. The WebUI exposes this on a per-dataset level. The websocket API can take arbitrary paths that are located under /mnt/
User - the owner of the file in question
Group - the owner-group of the file in question...
ACL manager in 11.3 Beta 1