X-Dimension
The X-Dimension is a universe. Virtually nothing is known about it, save that it is home to the X-Dimension Micron. Some people whisper that the dimension is home to strange, alien intelligences but they can't say for sure since probes keep vanishing when sent there.
Fiction
The AllSpark Almanac
Aero-Bot had an ill-fated five-person expedition from Yayayarst 201.11 Beta into the X-Dimension. He then resurfaced in the recesses of the Eshems Nebula of Tyran 609.22 Iota, barely alive and unable to remember who he was or what had happened. Transformers Animated: The AllSpark Almanac II
Legacy
The X-Dimension has never actually appeared in any fiction. According to the Micron Legend 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.
