The Shadow Realm’s three spacial dimensions are a, b, and c, as opposed to Mortal Earth, which has x, y, and z. This difference means that any travelers or non-natives to the Shadow Realm can easily be picked out by a trained eye. Some creatures, such as those from the planes of elemental fire and para-elemental smoke, can pass through more easily, as they have the same a, b, and c coordinates.
Another side effect of having different spacial dimensions is that a traveler’s body has to adjust to the new coordinates. This can cause any range of effects, but for travel between Mortal Earth and the Shadow Realm, most often it causes fainting.