(covers information from several alternate timelines)
A subspace fragment was a contained extradimensional realm which could be created as a byproduct of a subspace inversion.
In an alternate timeline in 2422, Jake Sisko attempted to use a subspace flux isolator to retrieve his father, who had been trapped in subspace fifty years previously. Timing the use of his device to coincide with the subspace inversion of the Bajoran wormhole, the device succeeded in creating a path to subspace, but instead of pulling his father out, it pulled Jake into a subspace fragment, where he was able to interact with his father, due to a subspace link which had been created between them at the time of the initial accident. Jake was soon returned to normal space, his attempt to rescue his father failing. The timeline in which these events occurred was negated thirty-eight years later when Jake committed suicide, thereby severing the link between them and sending his father back in time to the moment of the initial accident, where he was able to change history by avoiding the energy surge which pulled him into subspace in the first place. (DS9: "The Visitor")