There are a few comments which may be helpful in addition to the document Ray mentioned.
Both bs3 and bs2 curves are used in ACIS as approximations for freeform curve geometry. The number refers to the dimension of the codomain. A bs2 curve is a bspline curve in two dimensions (usually coordinate space for some surface). A bs3 curve is a bspline curve in three dimensions: these are used to define some geometries for edges (intcurves where the int_cur is and exact_int_cur) and to define approximate geometry for general freeform curves.
cheers
Eric
