>In offline rendering we usually just have some primitive (sphere, cube etc) that acts as the boundary
Right, using a SDF as a boundary condition is to discretely check it as an indicator function (if negative do x else do y). You can use basically any SDF to bound your procedural medium, including a procedural boundary (via domain distortion).
You can also apply a continuous blend instead of the discrete check (smooth step the level curve). This is preferred since discrete checks will often result in popping.
Right, using a SDF as a boundary condition is to discretely check it as an indicator function (if negative do x else do y). You can use basically any SDF to bound your procedural medium, including a procedural boundary (via domain distortion).
You can also apply a continuous blend instead of the discrete check (smooth step the level curve). This is preferred since discrete checks will often result in popping.