For the interpretation and difference between the project's albedo and the Bi-facial albedo, please see our FAQ "What is the effect of changing the albedo in the project ?
Now the mention of albedo in the Horizon part is not another definition of the albedo.
This is an interpretation - and correction - of the project's albedo in shading situation. If you have an obstacle (far shadings) rather close to your system, the albedo of the terrain in front of this obstacle may be diminished due to the fact that the irradiance is lower for this area (ie the terrain is in the "shade" of the sun), and this area is not so extended as an "unlimited" terrain.
For details, please see the Help "Project design > Shadings > Far shadings - Horizon
", especially the paragraph "Treatment during the simulation process