Windows Mixed Reality will Let you Setup Boundaries to use the Headsets & Experiences
Ever tries Virtual Reality with Oculus RIFT? The one with 4-camera complex setup which marks the boundaries of VR? Looks like Mixed reality Setup will have something similar except that there is no complex camera setup.

When setting up Windows Mixed Reality for the first time, users will be prompted to select between two modes.
Set me up for all experiences (recommended)
This will let you enjoy seated, standing and full room scale experience. It will create a boundary to help guide your movements when you can not see your surroundings wearing those headsets. The minimum area required for this is 1.5 metes by 2 meters of clear space.
I am guessing it will ask you to wear the headset, and then ask you to move around, and then click some buttons to set up boundaries. When you get close to the actual boundaries, you will be able to see lines to warn you.
Set me up for seated and standing
The obvious setup where you just want to sit and do nothing else. While this looks ridiculous, it might be much needed for those with little or no space. The classic example is let’s say if you want to use a program which needs to let you solve a puzzle with no movement required.
In case you want to set a boundary, if the need arises, you will need to go to Start > Mixed Reality Portal on the Windows 10 Desktop, and then follow up the setup process again.