A nebular flow is a region of space, specifically within the Sealed Kingdoms region of the Soul Nebula, where the density of interstellar gas is particularly thick and rich in isotopes useful in fusion processes.
Nebular flows tend to develop along paths defined by gravitational, electromagnetic, and electrostatic fields. Properly-equipped observation spacecraft can detect and map the flows by looking for the telltale emission and absorption spectra of the gasses, which also provides information on what specific gasses are present in a given flow. The reliability of this data is dependent in part on predictions based on physical modeling and signal lag between the observer and vessels looking to make use of the flows (see Natural Resources). Thus, it is important to not rely solely on data gathered at the origin point of a journey, but also to continuously update the data using onboard sensors. Multispectral scopes are often festooned around vessels like the ESCI Revelation for this purpose amongst others.
Fauna & Flora
While not inherently more likely to contain habitable worlds or other environments where life might flourish, nebular flows are commonly routes for the transmission of invasive species due to the increased amont of interstellar traffic. Asteroid mantises can be found in many systems along nebular flows, their spores having hitched rides aboard mining and exploration vessels. There are rumors that some extremophiles live within the currents themselves, surviving through chemosynthesis and hibernating through the long, cold gaps between stellar gravity wells, but any such species would have to be microbes with unusual physiologies and therefore difficult to detect through the usual means.
The major draw of nebular flows is that their isotope content makes them prime locations for interstellar travel routes. Ram-augmented inertial ramjets (RAIR) are popular drive systems for interstellar spacecraft in part because they can supplement internal fuel and reaction mass supplies with gasses drawn from their environment. Not all space is dense enough with the isotopes required to feed a RAIR, however, with nebular flows being a notable exception. While the amount of gas in a nebular flow is finite, it is also massive, meaning that only sustained use of these routes over millenia is likely to ever deplete them. The drawback of tapping into a nebular flow is that it need not follow direct routes to stars worth visiting, exploring, or colonizing; there may be the need to 'jump tracks' mid-journey, extending the mission duration in ways that might not be practical depending on the vessel's maintenance and life support requirements. Nebular flows tend to shift and meander over geological time, meaning that up-to-date astrogation charts are crucial for a mission looking to exploit their resources. If a ship is not RAIR-equipped, a nebular flow has more drawbacks than benefits; the impacts of gas particles against the hull of a ship travelling at relativistic speeds constitute particle radiation and have the effect of eroding material along all prograde surfaces. Even so, given the extreme distances and limited flight speed involved in interstellar travel, the added complications of using a nebular flow may be well-worth the resources it can provide.