Subspace bandwidth referred to subspace frequencies that could be measured using sensor technology. Signals of subspace particles from certain sources could be detected in specific bandwidths.
In 2371, Jadzia Dax scanned the lower subspace bandwidths around Deep Space 9, where she detected tetryon emissions from a cloaked Romulan D'deridex-class warbird. (DS9: "Visionary")
The subspace bandwidth also played a role in subspace communications. A diplomatic frequency occasionally used by the Vulcans in the 22nd century was in the lower band. (ENT: "Fallen Hero")