SSRD is a mod that allows players to see Sable physics objects further than the vanilla render distance. If you're building planes, cars, or ships using Create: Aeronautics alongside Distant Horizons, this mod is basically a necessity (especially in multiplayer).
The slider to change the Sub-Level Distance is located in the Sodium video settings menu, right under the normal Simulation Distance slider. The maximum distance allowed dynamically updates to match your Distant Horizons distance or the server's set maximum.
Partial Shader Support: Sub-levels can be seen through LODs when certain Iris shader packs are active.
Create Rendering Issues: Certain Create contraptions on sub-levels won't render outside the normal render distance (e.g., some propellers).
Sub-level Loading: For Sub-levels to be rendered, they need to be loaded. By defualt Sable unloads Sub-levels that aren't in loaded chunks, resulting in them disapearing. This behavior can be stopped by either using the /forceload command, or by using a mod like Create: Power Loader to force the chunks around around Sub-levels to be loaded.

SSRD rewrites the Sable mod to allow for physics objects (Aeronautics Planes!) to render further out than the normal view distance when using Distant Horizons