FrameMaker (standard) is based on paragraph definitions. While you can easily map your old paragraph definitions to SGML elements in order to quickly import legacy text, it's not the recommended way to do it. EDD's do all (and more) that SGML or XML DTDs can do, what XSL/FO does, what paragraph definitions in FrameMaker do. It's structure + style.
All EDD rules are based on the predefined Body TAG (which is a regular FrameMaker paragraph definition that you can modify btw). By default all EDD rules inherit definitions from this Tag. As you can see later, I suggest defining a very small number of base paragraph tags and build upon them.
Instead of let's say fixing a header as 18pt, you would make +6pt in relation to some default header.
EDD allows you to define rules for different contexts. That basically means that you can work with a much smaller tag set than in typical Framemaker. E.g. you can define rules for first and last elements in a list, for elements (e.g. titles) that have exactly or approximately such and such a parent, etc.