THE MATERIAL PLANES
Like pearls in an oyster shell, the multitude of planes sprawls out before the planewalker.  Each is a complete, and usually whole, world adrift on the æthyric waves of the Astral Sea.  The exact number and location of the material worlds is unknown, as is their diversity, but they all do share certain traits: each generally consists of a lot of ground or water with air above it, gravity that keeps people rooted to the world, and a thin, coplanar astral veil where incorporeal spirits usually reside.  Of all the types of planes, material planes are likely to be the most easily understood to the average layman.

THE VEIL
"Æethyrial Plane," "Near Astral Plane," "Spirit World," – all of these and more are names commonly given to what planar scholars call The Veil.  This is a thin astral-like plane that coexists with each material plane and acts as a buffer between it and the Astral Plane.  It is a plane between planes that shares traits of both, yet is neither.  Gravity in The Veil is not subjective, but its grasp does not as reliably anchor residents to the ground.  Solid objects in the material plane may or may not be reflected in The Veil, and even when they are, spirits can often pass through them.

Occupants of The Veil are completely invisible and intangible to those in the corresponding material plane, although some more sensitive people find they can catch glimpses of this hidden world.  Similarly, those in The Veil are only able to view a denuded and faint image of the material plane and cannot affect it anymore than those in the material can affect The Veil.

Game Mechanics:  A creature in The Veil is Insubstantial and Invisible to non-insubstantial creatures.  It can perceive that material plane, but at a -7 penalty, except in æthyric sinks, places where the spirit world and the material blur into one.  Occupants of a material plane (usually) cannot perceive the Veil (assume a penalty of -10); æthyric sinks, Acute Sense (Æthyric Only, -25%), the See Invisible (Spirit or Ætheric) are exceptions.

It is usually impossible for an entity in one plane to affect the other plane; assume such attempts take a -10 penalty, making them impossible to attempt; abilities that do not require activation rolls or Quick Contests simply do not function.  Exceptions to this include any trait with the Affects Insubstantial or Affects Substantial enhancements; Insubstantial Only or Substantial Only limitations; or Insubstantiality with Can Carry Objects, Can Affect Substantial, or Partial Change; as well as æthyric sinks (see below).

ÆTHERIC SINKS
At certain times and in certain places, a Veil and its material plane have been known to merge partially or wholly.  These places and times are called æthyric sinks.  At or during an æethyric sink, those in a Veil and its material world can more easily see and interact with each other.  The degree to which this is possible depends on the individual sink, but such locations and times are often held in high regard, either as sacred or unholy haunts, for only through powerful magic, miracles, or at an æethyric sink can an individual bodily travel from the material to the Veil or vice versa.


Game Mechanics:  Each æthyric sink has a rating from 1 to 10.  This is the degree of ease with which the two planes can interact, and residents can traverse from one into another.  An æthyric sink's rating offsets any penalties to perceive or affect the other side.  Perception and Perception-based skill rolls, as well as any activation rolls or Quick Contests for abilities intended to affect the other side benefit from this.  Abilities that do not require an activation roll or Quick Contest instead have an effectiveness equal to 10% x æthyric sink's rating.  This affects an ability's level, damage, protection, etc.

BETWEEN THE PLANES
Some entities exist between the Veil and the Material, able to perceive and affect and be affected by them both to an extent.  Many nature spirits experience this dualistic nature in one form or another.  Some are sensitive to the presence of the other side, while others can actually see, feel, touch, smell, affect, and be affected by the other side.  

Game Mechanics:  Those who are material (or Insubstantial) who can affect the other side physically have Æthyric Potency [20].  Those who can be affected by the other side have Æthyric Vulnerability [20].  Those who can perceive the other side have one of Cosmic, Extends to Insubstantial (+50%) on their levels of Perception; Acute Senses with either Cosmic, Extends to Insubstantial (+50%) or Insubstantial Only, (-30%); Æthyric Hearing [14]; Æthyric Sense of Smell/Taste [3]; Æthyric Touch [14]; or See Invisibility (Æthyric).Those who can communicate with the other side usually have Channeling (Æthyric Entities) or Medium (Æthyric Entities).

PLANEWALKING
Travel between material planes not only is possible, but is common among powerful sorcerers, spirits, and other entities.  Usually, this involves employing an existing gate between worlds or magically creating a temporary one (that usually closes as soon as the creator passes through it).  While it is dangerous in that one does not always arrive at the intended location – or plane for that matter – the main obstacles to embarking on interplanar adventures is the sheer power required.  Only masters of magic, extremely powerful artifacts, and nye-godlike entities can move about the planes at will, for no known natural gates between the planes exist.  It is rumored that it might be possible to travel from one plane to another by physically passing through the Veil into the Astral Plane and somehow navigating to the Veil of your destination, but the very nature the Astral Plane works against such voyages – there are no landmarks or a even concept of direction within the Astral Plane.

THE ASTRAL PLANE
The Astral Plane is the timeless infinity that stretches out between material planes and their veils.  Its composition is indescribable, though often referred to as æethyr.  Planar scholars believe this stuff to be the raw building blocks of all matter, for it is highly mutable and easily sculpted with magic.  Still, true creation using æethyr has yet to be achieved by any mortal (some presume the immortals of the Heavens and probably those of the Hells can create using æethyr).

Travel to the Astral Plane is most commonly attempted by mystics and shamans who project their soul into the abyss as a way to search for enlightenment.  Some claim to have found the Heavens and Hells there, but actual attempts to journey there via the Astral Plane have all resulted in failure.  Currently, the Astral Plane is held to be devoid of inhabitants.

THE SHINING HEAVENS
<Under Construction>

THE BURNING HELLS
<Under Construction>

The aetheric sinks are pretty cool, a refreshing change from the standard way that the equivalent of the Ethereal plane works. How does the scale from 1 to 10 function in terms of gameplay? What changes depending on that value?

Deliberation has occurred!  The ability for ethereals and materials to affect each other is generally prohibited, barring certain circumstances (powerful ghosts/spirits/wizards/etc.).  So the reduction in potency will be -100% + Sink Level x 10%.  In other words, multiply any damage (not injury), DR (from items as well as abilities), etc. by Sink Level x 10% to find its actual effectiveness.  If an ability requires a Quick Contest or success roll to function (e.g. afflictions, maledictions, etc. - not skill rolls to aim or apply physical attacks, such as Brawling, Wrestling, Gunenr (Catapult), etc.), apply a penalty equal to Sink Level - 10.

I will be ammending the original post with this information once it is better worded.

Can I go plane-hopping with my knight?

I would like to know more about the Burning Hells. What are they like?