PTRF Distance value is derived by calculating the difference between the "average PRTF amplitude of the 2KHz-7KHz range" (i.e. PRTF Size), and the "lowest PRTF amplitude value in the 8KHz-12KHz range". In other words, PRTF distance looks for the depth of the 10KHz notch, which is mostly responsible for cues regarding the angle and elevation of the sound source.
As a rule of thumb, headphones with angled drivers and large and deep enclosures have the best PRTF responses. That's why over-ear headphones tend to do better than on-ears, earbuds, and in-ears in this test.
What it is: Soundstage qualities are not inherent to the audio content, the headphones have to 'create' them rather than 'reproduce' them. They determine whether the sound is perceived to be coming from inside or in front of the head, how open and spacious the soundstage is, how much the headphones acoustically interact with the environment, and how strong the phantom center is. The depth of the "10KHz notch" of the headphone's PRTF, which is caused by phase cancellations at the concha. This quality is monaural and can be perceived even with one ear. This test does not apply to in-ears and earbuds, due to the lack of pinna interaction.
When it matters: This value is mainly responsible for the perceived distance and elevation of the soundstage to the out of the head. A small distance value may result in a soundstage that is perceived to be located inside the head. Larger values may help pull the soundstage out from inside of the head and bring it to the external head.
Good value: >13dB
Noticeable difference: 1dB