Knowledge (INT):
The Planes: Broad knowledge of the multiverse. The general types of planes, a basic idea of how is each plane, the general workings of portals and such, etc.
Inner Planes, Transitive Planes, or Outer Planes: Still broad knowledge, but more focused. Names of the planes, a general idea of the environment of each one, maybe something about natives or places.
Upper Planes, Lower Planes, Planes of Law, Planes of Chaos, or Planes of Conflict: More focused than the previous three. You can start to know some general information about specific things. Like how big or where is that or which city.
[Specific Plane]: The character have a chance (probably small) to know something specific from a specific thing.
As a general rule raise the DC by 5 for every broader knowledge used, and lower the DC by 5 for every narrower knowledge used. E.g. You determine that certain answer is DC 20 of Knowledge Upper Planes, if the character rolls using Knowledge Mount Celestia (or any other Upper Plane) the DC is 15, or if the character rolls using Knowledge The Planes the DC becomes 30.
Knowledge The Planes Group of Planes Specific Plane Direction nearest portal DC 25 DC 20 DC 15 Direction nearest portal to specific plane DC 30 DC 25 DC 20
Retry: No, unless new information that could lead to the answer is obtained.
Special: If you have 5 or more ranks in one knowledge you gain +2 to any rolls with all knowledges included in its spectrum (i.e. Knowledge Upper Planes, gives synergy to Knowledge Mount Celestia). Planar characters can use Knowledge The Planes, and Knowledge Home Plane untrained.