Vessela:

The BABOK is based on what is popular - not upon what makes sense.   Take Business Requirements vs Functional Requirements.   What the BABOK says is that if a requirement is high level and non-implementation specific, it is essentially different than a requirement that is not as high level and is implementation specific (i.e,  implemented within a specific system).  (Do not be confused by goals, objectives, and needs vs functions and processes;   a high level goal, need, or objective can easily be restated to be a high level essential function/process.  It is just a small matter of changing some wording around.)

Such thinking, especially for larger scale efforts, is very problematic as it retards the analysts ability to integrate all essential functional requirements into a comprehensive, integrated whole.   Data flow diagrams are proof that all essential goals, objectives, needs, functions, and process - high level, detail level, non-implementation specific and implementation specific - can all be tied together into a single comprehensive, integrated whole. 

Tony