In general, Enscape can be used offline, however there are some web-based features which can only be used when connected to the internet. These include:
Asset Library (Offline option available)
Please also be aware that you need an internet connection to activate your license. If you have a fixed seat license, you will only need an internet connection when you activate the license for the first time, and on the day it renews. If you have a floating license, you must have an internet connection any time you start Enscape or use one of the core Enscape features (Asset and Custom Asset Library, Enscape’s Material Library and Material Editor, and Enscape Objects in SketchUp).