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Basically BSP (Binary Space Partitioning) is an optimised game world based on volume, i.e. three-dimensional areas defined by a set of boundary coordinates. For Unreal BSP is 'negative' in that volumes are carved from the world, whereas idTech is 'positive', volumes are placed into a void. Both create blocks and areas. Models on the other hand are 'skins', they're essentially a collection of linked 2D plains manipulated into a shape resembling something.A Cube for example as a model, has 8 corner vertices that are able to describe 6 plains / 2D faces (three of which will share one vertex at a corner) that just happen to form a closed mesh. As a volume, those same verts and faces define a volume, an areas bound by those elements. The difference largely boils down to this: a mesh can have openings and gaps, a volume can't, they always have to be solid (if they are not they're no longer volumes). And this is basically where the problem lies, in converting models to brush volumes, each 2D face has to be given 'depth' or 'volume' to be meaningful in BSP world space. The extrusions that occur to facilitate that are where errors tend crop up (notwithstanding grid and other issues already mentioned). This means if you're making something specifically for converting to brushes you ideally need to structure the model so its made from blocks and volumes.