They represent a sequence of corner truncations, starting from zero in the upper left, to all eight in the lower right.
From the upper left, you're showing the one cube having one truncated corner, then 2 having 2 truncated corners - you've shown the variants with truncations at diagonally opposing corners on opposite faces, and with adjacent truncations on the same face. You're missing the variant with diagonally opposing truncations on the same face. In general, to be complete, you've got to explore adjacencies on the same face, and opposing truncs on both the same and opposite faces.
Your series proceeds with 3 truncs, 4, 5, 6, the one 7 and the one 8.
You can make all of these variants with the truncated trajber's edge piece, and the half-trunc edge piece.
And the triangle corner.
A nice exercise!
Sorry, I haven't bothered to figure out which others are missing...