mixed reality is a type of interaction model where virtual and physical objects interact with each other, and both can be controlled by the user physically - as opposed to augmented experiences where the user doesn't interact with the virtual elements directly.

the main characteristic of designing for mr is that the user is the centre of perspective and the source of all interaction - to have a working experience we need to design users actions & consequences.
it's about changing our way of thinking about interface - in mr a button is no longer a rectangle with a text on it, it can be a physical button you need to press down with your hand. this require us to come back and rethink interaction models & game rules:

Diegetic: Every digital element should be intrinsic to the environment
Familiar physical-world Interactions: Designing actions users innately understand
Real-world analogies: Leveraging pre-existing known concepts
Expected communication channels: Methods of communication that align with user anticipations
Ensuring universal compatibility for different kind of rooms and spaces