Hammerspace is a term used to refer to an invisible space where fictional characters can pull out objects larger than the physical space they are stored in. RPGs (Role-Playing Games) refer to this space as a Magic Satchel. In the Sonic continuity, many characters are seen pulling large objects out of nowhere (usually Chaos Emeralds and the Dragon Balls). This can be considered as sort of a "video game logic" that has no real meaning to it and is not important to the story or gameplay, but can sometimes happen as an element of gameplay.
In Shadow the Hedgehog, Shadow was shown having access to Hammerspace. He would often pull any Chaos Emeralds he had collected out of nowhere and then they would just vanish without any physical means of transporting them. Although in one scene he made a gesture where he appeared to be putting two Chaos Emeralds in his "pockets" although he wears no clothing.
Also, the guns Shadow carries also use hammerspace, indicated by the fact that he never needs to reload them.
Amy can pull out impossible amounts of Piko Piko Hammers out of hammerspace. This is even parodied in Sonic X where Amy simply holds out her hand a hammer appears in a puff of smoke. And in another episode, it appears in a flash of light.
Téa does has hammerspace of her shorts or her skirt. She first pulls her deck out of her pocket. Also, Téa takes a cartouche out of her pocket and gives it to Yami. Téa pulls her Good Luck Necklace out of her pocket and puts it on around her neck. Téa puts two Chaos Emeralds in her pockets. Téa puts the Two-Star Dragon Ball in her pocket after solved the mystery in the High-Tech House of Horrors.
Serenity also does has hammerspace. She hides the Devil's Eye in her pocket when Medusa tries to steal the diamond for herself. Serenity pulls her dairy out of her pocket to mention Lyoko and XANA. Joey asks Serenity to put the Four-Star Dragon Ball in her pocket.