Canonically, they don't exist. However, I seem to recall that such things
as postal unions and customs unions are permissible within the Imperial
framework, and the Imperium allows its member worlds almost total autonomy
provided that they (a) pay their taxes, (b) don't eff with trade*, and (c)
no you may NOT have nukes, those are MINE.

So stands the question: What inter-world organizations/pacts/et cetera are
allowed, and when (other than by specifically trying to declare one) does a
group of worlds "cross the line" from allowed agreements to Being A Polity?

* This is interpreted loosely - worlds can impose tariffs, but they can't
do so in a discriminatory manner - if frammistams from Potrzebie are
tariffed, frammistams from Furshlugginer must also be tariffed, and at the
same rates.  Ditto prohibiting the import of products; you can prohibit the
importation of widgets, but you can't prohibit the importation of widgets
from Wadgit without also prohibiting their import from Boff. Obviously,
just like the 'no slavery' clause, there are ways to Play Games without
Ticking Off Someone Who Would Have To Notice...

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