It's based on dead space in the silicone portion of the mask. Because the silicone bulb of the DreamPort is so small, it's normally used with the nasal pillow setting. That said, some end users use the nasal mask setting successfully. I would tell you if your AHI, Leak, and Pressures are staying about the same, not to worry about it, and use what is most comfortable to breathe on.

Thank you!