A geometry transform `T' is a geom that encapsulates another geom `E', allowing E to be positioned and rotated arbitrarily with respect to its point of reference. Most placeable geoms (like the sphere and box) have their point of reference corresponding to their center of mass, allowing them to be easily connected to dynamics objects. Transform objects give you more flexibility - for example, you can offset the center of a sphere, or rotate a cylinder so that its axis is something other than the default. T mimics the object E that it encapsulates: T is inserted into a space and attached to a body as though it was E. E itself must not be inserted into a space or attached to a body. E's position and rotation are set to constant values that say how it is transformed relative to T. If E's position and rotation are left at their default values, T will behave exactly like E would have if you had used it directly.
o-+ ode
o-+ dcollider
o-+ dgeom
o-+ dgeomtransform