Universal Mod Core allows developers to build mods against a stable API that is Minecraft agnostic.
In the past UMC has been packaged directly with ImmersiveRailroading as it is was the only mod using it. Now that the API is reasonably stable, it can be published as it's own mod. This mod contains the logic for each Minecraft Version / Loader that it exposes in a simplified way to child mods.
Check out the wiki page for more information!