X-Dimension
The X-Dimension is a universe. Virtually nothing is known about it, save that it is home to the X-Dimension Microns.
Fiction
The AllSpark Almanac
Aero-Bot led an ill-fated five-person expedition from Yayayarst 201.11 Beta into the X-Dimension. Transformers Animated: The AllSpark Almanac II
Legacy
Aside from The Allspark Almanac, The X-Dimension has never actually appeared in any fiction. According to the Micron Densetsu Year Book 2003, it was originally intended to be the subject of a mini-comic that starred the thirty Japanese-exclusive "X-Dimension Mini-Cons" released by Takara, telling the story of this parallel universe from the little bots' perspective. This comic never materialized, although in 2003/4, a 13-part mini-comic "Linkage" was produced as a pack-in for the Japanese Micron Legend (Armada) DVD-release, which retold the Armada conflict from the perspective of the Mini-Cons...but in the mainline cartoon continuity, rather than an alternate universe. It seems probable that the X-Dimension comic was dropped in favor of "Linkage" when Takara realized that the Armada cartoon was borderline incoherent a bit messy in places and needed someone to clean house more than they needed an alternate-universe retelling.
Beginning in 2009, several of the X-Dimension Mini-Con toys were repurposed to represent unrelated characters from the Shattered Glass "mirror universe". This was likely a deliberate homage to the X-Dimension toys representing characters from an alternate universe...but Shattered Glass universe itself is not the X-Dimension.
