Version 12 introduces a new tab for creating Groups in the Assembly Tree. The primary use of grouping parts together is for the Motion Simulation function: each group that you define can be given a different direction of motion.
However, grouping parts can also be useful outside of the Motion Simulation tool. A Group provides an alternative to the Assembly Tree for hiding or showing a large number of parts in a single action. While Groups can be used much like an Assembly Set, the Groups Tree offers color and transparency control which is not provided by the Assembly Set pane. And unlike Sets, a part can only belong to one group at a time, and parts can be moved between groups by using drag and drop in the tree.