Remember how the Doom Engine utilized Binary Space Partitioning to partition their maps? Well, the game Doom actually preprocessed and partitioned each map individually ahead of time with Binary Space Partitioning, which was a very time-consuming process at the time and even could take up to 30 minutes per map. However, Doom had the flexibility of sorting walls, determining the positions of sectors, and even collision detection. The walls in Doom could NOT move.
However, Ken Silverman's Build engine removed this limitation of static walls by not implementing preprocessed maps. Build instead relies on what is known as a Portal System. Portal Systems are really useful and fast for eliminating regions of world geometry that are not within the player's view. The idea behind portal systems involves dividing the game world up into different zones that are connected by portals. Essentially, portals are represented by convex polygons from which one region (zone) of the world can be seen from another region (zone) of the world. Think of this as if you are standing in a room (a zone) and you are looking into the next room (another zone). The portals would be the doorways leading into the next rooms! Since game worlds can become massive, rendering the entire world all at once will really slow down your computer/device or possibly even crash them! However, since only certain regions of the world within the player's field of view need to be rendered, we can GREATLY speed up graphical processing and rendering! I'll provide a picture for extra clarification!
Portal System with Culling of regions to what is visible through the player's eyes. Everything that is white is not rendered!
Retrieved from https://www.panda3d.org/manual/index.php/Portal_Culling
Very detailed Portal System diagram., which is considered faster than BSP!