Is B-REP a ready to print mesh
a) about 


  Is B-REP a mesh that only represents a boundary [..] ?


B-Rep contains more information, see here:


  There are two types of information in a B-rep: topological and
  geometric. Topological information provide the relationships among
  vertices, edges and faces similar to that used in a wireframe model.
  In addition to connectivity, topological information also include
  orientation of edges and faces. Geometric information are usually
  equations of the edges and faces.


b) about 


  and therefore ready to be printed


it is need not only check that the model is correct (no lack of a face, no wrong face normals, ...) but also that is valid for a 3d printer (no "floating" parts, addition of supports, ...).

Finally, recall the model is not what drives the printer, the printer is controlled by the machine instructions, usually gcode. Thus, the usual evolution is: parts description (.scad, ...), model format (.stl, ...) and machine instructions (.gcode).