It is no secret to the residents of Maycomb that Calpurnia is not just hired help in the Finch home but a mother surrogate for Jem and Scout. In the courtroom scene, moreover, Calpurnia's close affinity with the Finches is publicly displayed.
Symbolically, Calpurnia represents both the bridge and the chasm between the white and black communities. At the trial, the whites and the Negroes sit separately except for them. Calpurnia in her natural dignity shows that she is not ashamed to be black nor is she ashamed to identify with whites. She would be the truly integrated Negro, if only society would permit.