A feature is a unique combination of the location of infrared markers (or locations of reference bars, if markers are combined into elementary patterns).
Each feature occurs only once in the Environment and is not repeated as a result of rotations, reflections, and other transformations of position.
Features are used by the tracker to determine its position within the tracking area.
Take Floor Tracking Area as an example. Let's place 2 reference bars (6 IR markers) on the scheme, as shown in the figure below. These markers form the first feature.
Let's add 2 more reference bars and arrange them in the same way. The original feature will be repeated and therefore will will no longer exist as a unique feature. The combinations of IR markers are repeated, so the tracker cannot unambiguously determine its position.
The same will happen if we rotate the second pair of reference bars by 180°. The location of markers will cease to be unique, and instead of two features we will get none.