'[B]' sprites are not in fact sprites at all but rather 'markers' used in the Keen Galaxy games. They are a special value in the infoplane (sprite) layer of the level that act as 'barriers' to certain sprites. This is part of the mechanism behind Platforms and switches used in these games. In Keen 5 a more complex series of markers are used that can change Platform's directions instead of simply reversing their direction as in other games.
As the [B] sprite is not a sprite in the traditional sense as many of them as desired can be added to a level without causing an error. They cannot be placed over animating tiles however. (Animating tiles overwrite the infoplane.)
Any patches relating to [B] sprites will affect the sprites that react to them and are therefore are on those sprite's pages.