Microsoft HoloLens Developer???s Guide

Placement

Since we do not know what the area the user is in looks like, we have to scan for this. By scanning and using spatial mapping we can make educated guesses about where things are. We can deduce where the floor is, where to find walls, and so on, by processing the data the spatial mapping engine gives us. I will tell you more about this later on, but for now, just accept that we can do this. However, this is not always enough. Sometimes you will want to give the user control of the layout of the virtual world.

If we are writing a holographic version of chess we want the user to be able to place the board on the table. We can find out where the table is, but we do not know where the user wants to sit, especially if it is a large table. ...