Shadow Veil
AKA "the Veil".
Shadow wraps around the Mortal Plane, isolating it from direct contact with the other planes. But wrapping around the deeper layers of Shadow itself is the Border Shadow, acting as a planar lubricant. And Border Shadow itself has two meaningful layers--the Verge and the Deeps.
The Veil is the "membrane" that separates the Verge from the Mortal Plane. It prevents mortal, physical entities from accidentally crossing over into the Verge, while also adding resistance to crossings in the reverse direction. Any teleportation or conjuration spell requires crossing the Veil, as does any ability to become ethereal. As does any manifestation of divine, demonic, devilish or angelic power in the Mortal more directly. Priests who petition their patrons, clerics awaiting promptings from their gods, and demon cults performing sacrifices all interact with the veil. Even incorporeal entities, such as the spirits of the dead, must pass through the veil to reach Shadow.
Pure aether, bereft of any consciousness, can pass almost entirely unhindered by the veil. As a result, spells and effects that move aether around, as well as worship, pass through the veil with only the most minimal of disturbances or resistance.
Structure of the Veil
The veil itself is often thought of as a living organism--it changes and adapts to conditions and is self-healing against small wounds.
Some scholars suggest that the veil can be thought of as comprising multiple systems, including the Between (the medium in which everything else happens), micro-portals, macro-portals (the two differentiated by size and stability), and even an "immune system" of sorts, although this latter is exceedingly poorly attested.
Passing through the Veil
Bodily passage of a tangible creature through the veil is difficult. It generally requires finding or creating a portal large enough to allow passage. Some of these, such as the Moonpaths, are persistent. Others are ephemeral. Controlling where (and when) you arrive is also difficult. Most "normal" spells use workarounds.
Divine Guidance
Even something as simple as a cleric receiving guidance or power from their deity requires reaching through the veil. For the most part, the veil has minimal effects on this. However, it does cause ripples and, in areas where the veil is exceptionally thin, risks further damage. Greater manifestations (dreams, actual messengers) cause exponentially more disruption and have become quite scarce during the Time of Troubles to avoid cascading veil disruption.
Psychopomps and Incorporeal Creatures
The Catocracy has to patrol the Verge and Veil routinely to find lost souls. They do so by navigating micro-portal structures within the veil, often called "mouseholes". This they can do because cats are particularly flexible, both physically and metaphysically. These portals are purely ephemeral. Incorporeal creatures do the same. As a result, a particularly "thick" veil can impede souls from transiting into Shadow; this is used in several arcane workings to trap the disembodied.
Conjuration Spells
Most conjuration spells don't actually create large portals. Instead, they create or find a micro-portal, large enough to pass an incorporeal Name and then build a substitute body-shell out of aether at the destination. This also produces a beneficial side-effect--creatures conjured in this fashion do not die if their shell is destroyed; they are simply banished back to their home plane.
Demonic and Undead
These types of effects (summoning demons or creating undead) involve pulling jotnar through from the Abyss. This is inherently corrosive to the Veil and the tears formed in this process are slow to heal even under the best of conditions. As a result, every demon or undead summoned raises the likely-hood that more will leak through spontaneously. And as the veil reacts to form scar tissue, it raises the chances that any souls that die in that area will only find their way into the Waste to become prey for demons.
Short-range Teleportation
Spells such as misty step and other short-range teleportation (ie that that happens within a few dozen feet) don't actually pierce the veil. They simply skip the user along the surface of it, utilizing the leakage of Shadow's space-warping effect to bend space. As a result, this is only marginally affected by the Aether Flood and is possible even where most planar movement is not possible.
Long-range Teleportation
Long-range teleportation (e.g. teleportation circle, teleport, etc) do require piercing the veil. Effectively, the spell creates a tunnel through the Verge, making a single step into a trip of arbitrary length. Tearing the veil in this fashion is both prone to disruption due to local aetheric conditions as well as damaging to the veil itself (although small amounts of travel don't cause lasting damage).
Plane Shift
Actually physically translating between planes is the largest form of veil breach. The spell by that name actually looks for existing portals leading to approximately the correct location and widens them instead of tearing new portals.
The only exceptions here are transits to demiplanes (which exist within Shadow itself) and summoned creatures (such as devils) voluntarily self-banishing, relinquishing their shell and returning home. These are fairly benign.
Veil Thickness
The veil is not of constant thickness (ie resistance/durability) everywhere. It varies both with location (including down to small-scale effects) and with time. Both too thick (ie hardened by scar tissue or magic) and too thin (weakened by various forces) are problematic.
