A Squeak distribution is simply a base Squeak image being regularly maintained by some project/community.
There is of course the official Squeak base image, the one you get when you download Squeak, but also a few "forkish" images that are being actively maintained under other names, like for example:
Croquet.
Squat.
Squeakland.
Small-land.
This page is linked from the "Squeak distribution" category on SqueakMap which has sub categories for these distributions that then can be used to categorize packages to be intended for that distribution.
If the package is for "official regular Squeak", then just pick "Squeak".
But if you publish a package that is meant for Croquet - then you select the Croquet category for that package.
Note: Currently there are no version categories for these images. Croquet is just Croquet. A better model will be used when we add dependencies to SM.