I have looked through the game again, and I believe we can safely conclude that Phantasma is a universe. That would make the feats regarding it 3-A/Low 2-C. It would be nice to hear what staff say about this, but we should go forward for this in mind.
There are some things to note that make this case:
- Celeste explains Phantasma's structure here. She calls it similar to a fictitious realm in the sense that peoples' desires are realized in Phantasma. Of course, it's not actually fictitious, given this was her simplifying its nature. This seems to compare the scope of Phantasma as a world to the main universe. Furthermore, Anima Mundi treats the two as counterparts too.
- The contents of the planes are treated as physical spaces. When Kevin and Ries first arrive in the Hermit's Garden, they observe its huge size by looking at the starry sky before them, saying it goes out as far as the eye can see. Multiple characters even comment on these being stars too. Josette talks about formed constellations in Hermit Space, and Renne directly mentions the stars but how she wishes she could see the moon. The planes are also different spaces from one another, so it's not like they would share the same night sky anyway. You need portals to travel between them because they are isolated from one another.
- We've already talked Hermit's Garden being 4-A in size, but the other planes have similar things going for them. The Jade Corridor (First Plane) has its own starry sky background, Le Locle (Fourth Plane) has its own starry sky, the Wastelands have their own starry sky as we Anima Mundi literally fly out of the atmosphere and enter space itself with the sun also in the background, and Grancel switches from day to night, showcasing it follows a lunar cycle. This demonstrates a minimum 4-A size for the subplanes within Phantasma. What's more interesting is that Celeste directly mentions that Phantasma could host countless amounts of these planes, presumably of a similar size, as well.
- The realm itself appears to be spatially and temporally distinct from the main universe. Kevin and Ries mention multiple times that Phantasma is spatially enclosed, and one of Phantasma's gimmicks is that it has completely separate universal rules and laws governing it. This part is admittedly weaker evidence, but Julia also notes that the sense of time in Phantasma appears asynchronous to the real world, given she can't precisely discern how much time has passed.
The standards we have to meet can be found on
this page. I believe Phantasma meets enough criteria to satisfy the universal guidelines, personally. It has evidence showing it is a temporally and spatially distinct location, contains many (up to potentially countless) subplanes that are 4-A in size, and it's treated as a counterpart to the "real world" like the mirror world bit mentioned in the guidelines.
Some of the statements that come from random character dialogue might be doubted, but I believe it's safe to use. Many characters openly mention talking to Celeste after she is reformed. The consistency of these comments still follow after she is reformed, and she had already gone about correcting any misinterpretations of Phantasma that the cast had. If ideas like a different flow of time or the stars being actual stars were erroneous, it stands to reason Celeste would have corrected these alleged falsehoods. However, she does not refute these observations at any point.
Also, something to consider is how consistent this would be. The SC cast was able to fight Weissmann fused with the Aureole, which itself created and sustained Phantasma prior to its disappearance. It makes 4-A to Low 2-C very consistent fairly consistent for the Sky Trilogy, at least by the end game of SC.
