As in: what goods you can request from a liaison vs. what you can get from a caravan.
So I genned this world with no dwarf civs having access to bauxite. That is, I can not request bauxite stone from any civ--could not, my dwarf liaison died decades ago. A few years ago, I found the dwarf caravan had brought me a bauxite block, which I snatched up and have since very carefully managed (it was almost used in an artifact!).
It just hit me yesterday: how did they get a bauxite block?
Is there some known mechanism at work here? Perhaps the dwarfs can trade with another civ that has access to bauxite, and it was transferred to them. Or maybe only stone, not block material, is affected by access limitations. Maybe it's all random unless you make a request. But, then could I possibly get a random bauxite stone from the dwarfs someday? Maybe the death of the liaison affects what a caravan can bring. Anything?
I'm pretty sure I couldn't request bauxite blocks, but I can't check as my liaison is long gone. I'm 100% certain I couldn't order bauxite stone, as I tried multiple years in a row while I could and it was never listed.
I'm not very good at searches, but I have poked around the wiki, the forum, and the Google and found nothing on this besides some passing mentions about civs having access.