You answered your own question pretty much Andres.
It identifies the basic platform that the Bronco was delivered in from the factory. Wagon (full hard top and doors), Pickup (half-cab), Roadster (convertible with no doors), etc.
It also takes into consideration whether the standard or heavy duty springs and probably brakes were included. In other words, the one in the example above is a U-150, so that's a full hard top Bronco with standard suspension.
There are some inconsistencies though, as my own '71 is a U-150, but came with both the heavier springs and the larger brakes. So there is probably some variable in there that makes would have to be added to make it an official HD Package.
Paul