Bryce Units (BUs) are in the Bryce artist guide as:

Bryce Units Bryce maintains an invisible, absolute, infinite 3D grid internally. This grid is comprised of 3D cube increments, each of which is 20.48 x 20.48 x 20.48 "Bryce Units" (or BUs) in size. Most of the primitive objects (spheres, cubes, etc.) are created at the same size as a 3D cube.

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