There are several improvements and additions to assembly relationships.
Better key point selection.
Enhancements to axis selection.
Enhancements to the connect relationship. See the Connect command (Assembly environment).
Changes to the sketch selection UI for the mate and align commands.
Tangent relationship processing with zero offset.
Ability to create a rigid set of assembly components.
See the Rigid Set command.
Ability to create a Path relationship.
See the Path command.