That's pretty close Zoltan, but there's so much more to the drama:
Here's the story from the trenches, from someone who was actually there: me.
The prefix is indeed a throwback to the old 400 days. Remember the only product JDE had back then was the World product and that means everything we wrote was on the 400 which means everything was RPG. And even more significant, everything was RPG III which had certain limitations. For one thing, when you made assignments to table columns the language did not distinguish between which tables the columns belong to. Well it did, but not directly because the only way it would distinguish between tables is by the column names themselves. So, if I had table A and table B and they both have a column named TIME then my RPG program could not distinguish which table the TIME column belongs to. But, if I make a conscious effort when I make the table to give the columns a distinct name like TATIME (for table A) and TBTIME (for table B) then I can work directly with the appropriate table column in my RPG program. Thus, JDE came up with a standard for creating what we call the prefix and of course when One World came out it was coexistent with World, so it had to be brought over to One World too.
Now here's the super scientific method that we used when picking a new prefix: first we would look to see if our desired prefix existed anywhere. If it did not then we would just use it. Now it was pretty rare that we could come up with a prefix that no one had thought of before, because there were only two letters you could use. So, when you found the table, or set of tables that already had your prefix you had to make a decision. If the tables with your prefix were in some far off product that your table would probably never appear in the same program with (like real estate, or payroll) then you took a chance and just used the prefix. If it was in something close that stood a reasonable chance of actually appearing in the same program as yours, then you had to pick a new prefix and start the process over. And that was that, you really just took a grand guess and jumped.
In the brave new world of Enterprise One and the way the tool distinguishes between tables and columns and the use of SQL the prefix idea has become obsolete. However, the present tools work with prefixes and we're stuck with them despite their having lost their meaning altogether.