[18] A small number of Transformers: Energon Built to Rule sets had a limited, test market release, but the entire line performed poorly, so it was dropped in its entirety in 2004.
[citation needed] There are some significant differences between the Armada and Energon sets, though both are based on the same basic premise.
Each Transformers kit is centered on a "Trans-Skeleton", a very simple humanoid body that folds up for vehicle mode without dis-assembly.
For Armada, the Trans-Skeleton was a very broad, flat solid plate with thin, stick-like limbs attached to it, which led to very awkward-looking robot modes.
The structure of the Trans-Skeleton was changed for the Energon characters, to a much more solid skeleton with larger, blockier limbs.