So, if I understand this correctly, despite there being numbers explicitly for Agfa Vista 200 and 400, the DX codes actually identify them as Fuji C200 and Superia 400?
Its because of the complex history behind current "Agfa" films. In 2005 Agfa-Gevaert sold off it's consumer photo division into an independent company ("AgfaPhoto") via a management buyout, and made long term agreements to allow this company to use the Agfa trademarks. Within a year though, this new company had filed for bankruptcy and ended up in other hands essentially as a holding company for a "badge"; many products may be branded as Agfa (digital cameras, photo frames etc amongst other things), but they don't make any themselves as they just license the name.
Ever since then the actual films have not been produced by Agfa in practically any form, they've been produced by Ferrenia or Kodak and more recently exclusively by Fuji. The films may be named the same as the previous Agfa produced products (e.g Vista, Precisia etc), but they're actually just other films re-packaged in Agfa Photo branding (If you look carefully, you'll see that they've actually named Vista as "Vista+" to differentiate it from the original Agfa produced Vista). The only very slight almost exception to the rule is with APX B&W films: they're originally manufactured by t
he "Specialty Products" division of the original Agfa-Gevaert who make still motion, aerial,scientific/technical etc type films. Lupus Imaging (who have been granted a license to use the Agfa name and also are the originators of Vista+ and Precisia) then buy it as bulk stock, which they cut down and re-package (the same as Rollei do for most of their films) for sale.
The bottom line is that although it may look almost identically like the original Agfa films on the outside, on the inside it's an entirely different film and this is why the DXn numbers correspond to other Fuji films: they're originally made by Fuji!
The most useful part about the DXn number is simply that it allows you to identify a film, no matter how it many be branded. The barcode on the film allows automated lab processing machinery to identify the film type exactly, and process it accordingly (especially useful with B&W films as they have no standardised processing time for every film); the number beneath it (the DXn number) corresponds to the barcode in a numerical form and this number subsequently corresponds to two DX Numbers, Part 1 and Part 2 (I'm not sure of quite how, but that's why the simulator is useful as it allows you to extract the parts). Part 1 identifies the film manufacturer (they were assigned groups of numbers each every few years), and Part 2 identifies the individual film type when paired with the other number.
The same system actually corresponds to the edge code on negatives, so you can use the simulator to find what a film is even if it doesn't say anywhere (hold it up to the light and you'll see a little pattern of bars down the edge of the negative, you can then copy the pattern into the simulator).
" h'mm it's only Agfa