BG,
Let me give you two examples:
The Trojan Reaches were "officially" published in TD20, under DGP copyright. However, it was also published prior to that under a different copyright, portions of the data was published in AotI, and some of the subsectors were published in CT Adv4 and TNE RS. Consequently, since all of the data in TD20 was also produced elsewhere, that data is fair game.
On the other hand ...
MT World Builders' Handbook (and its predecesor GS/GC) used their expanded rules on Regina. One neat tidbit was that, on Regina, only persons born on Regina can vote. If you emigrate to Regina, you can't vote. Ever. But any children born on-world can. This piece of data appears no where else, and therefore, cannot be used. (Even though I would *really* like to.)
[Hunter: please feel free to correct me if I am wrong, and delete the message if I am completely wrong. But I think I have this right.]