Hey there moojjoo.

I've no intention of starting a flame war here, just maybe another 20 years in the industry could help answer the question.

In lieu of that however...

A UI has a set of design principles, people think in a relatively limited way, on average, about the task in hand, so there should be no extraneous data or tasks presented by the UI. I'm inclined to say that this is slightly more of an art form than a science.

The business logic has any number of sets of design principles from which to choose, but this is where the real engineering comes into play, ensuring that the minimal set of data on the UI is able to be translated to and from the entire dataset, via a set of proveable rules, with which the application is dealing.

Then the real art form comes in to play, and I'm talking about the data architecture here, not the ramifications of the architecture on the disk RW issues. That may well annoy a few DBA's reading this, but after being involved in the industry from back into the history of IMS/DB and with RDBMS's since DB2 1.2, I've developed the occasional opinion along the way.

I've very strong opinions in favour of 5-normal in a commercial environment. Oh dear, heresy again. That's at the table level, don't care what you do at the application side of an SP, just DO NOT let the the analysts and programmers on to the DBMS side of the SP unless they are able to talk cogently, at length, on the fly about 6-normal and it's applicability in class persistance.