The XYH-6 X/Y capsule comes standard with the H6.
The capsule adds two matched high-quality unidirectional microphones to your H6 or H5. This kind of microphone is most sensitive to signal coming from directly in front, and less sensitive to signal coming from behind or from the sides; in technical terms, they are said to utilize a cardioid polar pattern. These mics have the largest diaphragm size (0.579” = 14.6mm) of any ever available on a portable recorder.
Playing back stereo X/Y tracks in mono results in a fuller sound than you would get with one microphone, but if mono compatibility is critical (for example, in television production), the Mid-Side (MS) mic technique can be a better choice.