Subspace

Subspace is one name used by Transformers to refer to the extra-dimensional "space" that exists between realities. Although more properly known as "Transwarp", those who use the "subspace" term do not commonly realize that the two concepts are related (or are not even aware of the existence of Transwarp in the first place).

Specifically, subspace, referred to with that name, is used as a medium for faster-than-light communication, and as an extra-dimensional storage space for Transformers to keep their weapons and other personal paraphernalia.

Force of Habit
Some Transformers have access to an "extra-dimensional storage pocket" for their own personal use. As their name implies, they are extra-dimensional pockets that are used for storage. To retrieve something, a Transformer "taps into" the pocket, thereby "summoning" the object.

OOC
The idea that Transformers can store equipment or portions of their body in an extradimensional space has been popular in fandom for decades, essentially arising as an answer to perhaps the most famous question of all: "Where does Optimus Prime's trailer go when he transforms?" When the Autobot leader transformed into robot mode in the Generation 1 cartoon, his trailer would almost always slide out of the frame, not to be seen again until he transformed back into truck mode, at which point it would come sliding in to connect back up with him. Rather than believe that the trailer was constantly following Prime around like a puppy, conveniently perpetually out of shot, fans warmed to the notion that it was, in fact, being shunted into this extradimensional "subspace", where it remained until Prime summoned it back.

This "subspace" theory also helped to explain where characters' weaponry disappeared to when they transformed into vehicle mode, and happened to line up well with very occasional instances in the show, which featured the animation "cheat" of having some characters' weapons appear or disappear in a simple flash of light. This technique occurred as early as "More than Meets the Eye, Part 2", when Ravage transformed into cassette mode, and his hip-mounted missiles vanished in a flare of purple energy. Despite this early precedent, the effect was very rarely used in the American series, occurring only a few more times: with Kickback in "A Plague of Insecticons", with Cyclonus in "Dark Awakening", and with Blurr in a Season 3 commercial bumper. Conversely, later Japanese series Super-God Masterforce and Victory used the effect like it was going out of style, with characters regularly manifesting weapons of all kinds in their hands, with light flashes of all colors, including white, golden, blue, green, purple and pink. Notably, in "Resurrection!? The Decepticon Fortress", the idea of plucking weapons out of thin air was actually made into a plot point when Deathsaurus was unable to summon his weapons due to depleting his personal energy supply.

This "subspace glow" effect also saw some limited use for other means in the American series. In "More than Meets the Eye, Part 2", when Jazz retracts his hand and replaces it with his grappling hook, his entire forearm glows orange in-between. In one particularly notable instance from "The Ultimate Doom, Part 2", the effect was even applied to Prime's trailer; although it still rolled in from offscreen, rather than materializing out of nowhere, it was shown to be glowing with yellow light.

In a similar vein, subspace also helped to explain the assorted cases of size changing in the cartoon series, with the notion that characters who shrank when they transformed, like Soundwave, Megatron, Perceptor and others, shunted their extraneous mass into subspace (because their mass was going somewhere, since they became proportionately lighter when they shrank, rather than remaining the same weight). Again, the idea intersected well with one particular instance in "Transport to Oblivion", which saw Soundwave's body emit a "subspace glow" as he transformed and grew back to robot mode (although admittedly, out of the scores of times characters changed size when they transformed, this was the only time such an effect was employed). While the original cartoon never addressed the concept of size-changing, numerous explanations for the process would be featured in later continuities (see the concept's own article for details)

Although the subspace theory originated with the cartoon, evidence to support it could even be found in the Marvel comic series; specifically the 1986 story "Dinobot Hunt!", in which Jazz was shown to open a panel on his hip and remove a tiny version of his gun, which subsequently grew to full size. Years later, even Beast Wars would feature an animation cheat that subspace could be used to explain, which would involve characters drawing their weapons by reaching behind themselves, despite the fact that any shot of the characters' backs did not show a rear holster or anyplace to keep a weapon.

Regardless of the popularity of the concept in the fandom, subspace remained firmly in the realm of fanon for years, until it finally received its first named appearance in fiction when Hirofumi Ichikawa featured it in his Binaltech storyline in 2005 and used as a means for faster-than-light communication. A few years later, Greg Sepelak and Trent Troop canonized the whole shebang when they featured subspace by name in the Transformers Collectors' Club prose stories "Gone Too Far" and "I, Lowtech". These stories officialized the long-standing concept of subspace being used as personal storage space for Transformers (a previous story by the two authors, "Force of Habit", had mentioned extra-dimensional storage pockets without explicitly referring to them as subspace), while at the same time tying it into the cross-dimensional Transwarp space from the Beast Wars animated series.

Following these canonizations of long-standing fan concepts, about the only thing subspace hasn't been used to officially explain is size-changing. Even then, IDW Publishing has made use of the idea of extradimensional mass-displacement to explain characters' size-changing in their Generation 1 timeline. So we're just waiting for you to clarify where the mass is getting displaced to, guys!