The base is the principal organising tool in [B/D]. Elements, the belief carrying quantities, are organised into bases, which thus become the main focus of attention. We consider elements to be bases in there own right, and from the point of view of adjustments we adjust bases rather than elements, and our summaries refer globally to these collections of elements rather than to the elements themselves (although, of course these details are available for the elements individually).
The BASE: command is used to declare bases, and the XBASE: command is used to remove them. The LOOK: command can be used to inspect some or all of them. See §6.2 for details of specifying collections for the definition part.