The Subselar Boundary of the Castraversi

The Subselar Boundary is Yirimoth designation for a region that appears on Frameworks marking the space situated between the Planar and Selar of that Framework. From a different perspective, it's similar to bodies of water on Planarscapes, and tends to form due to specific circumstances unique to each Framework.
On the Castraversi, this anomaly forms a particularly unique route of travel between the super-Planar and the many Selars it has created. Some say this is due to its very unique creation circumstances, but any desire to learn more is hindered by Ars occupation. Subselar Boundary research is still a new principle among Yirimoth academics and as such any understanding on how they form tend to be found from several sources and with no standardization on these principles of research.
To the Udo that have populated the Planars and Selars of the Castraversi, the Subselar Boundary is a very important region of travel and a food source as well as a huge part of their cultural understanding of the Yirim. To the Udo of the Castraversi the Subselar Boundary is more intrinsic to their cultural identity than it may be in other Frameworks. 
Examples of this cultural infusion from the Subselar include the collection and distribution of several species' exoskeletal remains for avocation purposes as well as ritualistic and cultural markers. It is actually very uncommon for Udo of the Castraversi to not have some sort of jewelry or clothing implement that isn't based on what they find from the Subselar Boundary. 
Another more defined example of this cultural infusion is the marker of collecting Spikes from the The {Spike Lord}. Often the feat is considered next to impossible, however, due to the rarity of the act, the Spikes tend to be much higher in value and can be used both as a status of power for a warrior or a marker of wealth for the Udo who seek to gain prominence through materialism.
Not just a variety of wildlife, the Subselar's unique structure plays host to a wide diversity of plantlife, adapted to the fluctuating and constant change of forces to the area. This takes the form of some species that are free-flying and can float by evolutionary methods throughout the medium of the Subselar or those that find ways to stay stationary, either by creating the anchors themselves or by using other species to do it for them.