The Breath Master List was based on one thing and one thing only: how fast the Rune Breath Secret Realm was cleared. Any existence younger than the age of 51 and within the F-Grade was allowed to be on it. Though there was no universal list, the logistics of documenting so many Secret Realm entries across so many quadrants were far too great.
Instead, this list was based by District, while there were probably hundreds of Districts in just a single galaxy and about four to five solar systems to a District.
To Earth, it seemed as though most solar systems were lifeless, but this was only because most of the universe was closed off to them due to their failure to complete the Summoning.
In reality, of the thousands of solar systems in the Milky Way, all of them had at least one planet with life on them, maybe multiple planets and moons at that.