With T5 adding a pair of "pre-" codes to the Agricultural and Industrial groups, there are four steps in each. How the codes impact the trade rules strongly imply that the two end points (Non- and either Ag or In) represent notable net importers and net exporters, while the no-code and pre-code steps are a little harder to easily distinguish.
Pre-Industrial implies a preparatory stage when a world, much like a full Industrial world, is soaking up raw materials at a mad rate. The difference is that the world is not processing for export, but instead processing for internal use, be it infrastructure, population support, or additional steps in the processing chain. In a setting where world development can be watched, a pre-Industrial world is a small boomtown of almost a billion people, looking for and rewarding its entrepreneurs, potential oligarchs, and technological visionaries.
Pre-Agricultural is a little harder to pin down, as the conditions for the Ag code sit in the middle of the Population range instead of one end. Pre-Ag is either too few or too many people, so a world either lacks the manpower to manage the resources, or is starting to outrun its production ability with population growth. If a Pop 8 world wants to regain its Ag rating, either it has to encourage most of its 18 year olds to go offworld for college (and not come back), or classify anyone over 50 as a farm animal.