In the D & D 5e group I play in, we are currently wondering how much our characters (all levels 7) would know about the planes in the Forgotten Realms campaign (we play SKT, though otherwise for the Question is not relevant).
The knowledge in question concerns only the foundations of the Great Rad Cosmology, as described in PHB in "Appendix C: The Levels of Existence," pages 300-303.
Detailed information about a particular level, planar creatures, or planar politics (eg, the Blood War) obviously requires either a review of the knowledge or a background story, which is why the character knows this particular type of information (eg, a Githzerai character, who was sent from his apartment in Limbo) to combat Mindflayers on the material level, would obviously know about Limbo and its operation.
Chapter 7 of the PHB, "Using skill evaluations," also states that the Arcana skill is meant for level checks (see quote below), but not whether you need to know that levels are a thing of the past.
Arcana, Your intelligence test (Arcana) measures your ability to remember lore […] the levels of existence and the inhabitants of those levels.
But suppose that there are three different types of people for this question:
- People with knowledge of the Arcana skill
- Magician without knowledge of the Arcana skill
- Non-magician without knowledge of the Arcana skill
- normal citizens
Which of the following facts about the Great Wheel cosmology are known to average representatives of these 4 types?
- Existence levels are one thing / there is more than just the material level
- the general layout / layer names as indicated in PHB on page 303
- the distinction between demi, transitive, inner and outer planes and how they differ from each other
- Methods of planar travel
Evidently, magicians – even without Arcana knowledge – at least have basic knowledge, otherwise it would not make sense to spells such exile (lvl 4), Contact other aircraft (lvl 5) or just shift (lvl 7). Or would you not be able to cast these spells if you do not know about planes?
I'm mainly interested in clues in official sources, whether it's rulebooks, novels, developer tweets or anything else. Since my group plays 5e, sources for this edition would be preferable, but ultimately sources are fine for each issue.