A subspace layer is a region of subspace.

In 2372, USS Voyager encountered Suspiria and learned of Exosia, which was described as a "subspace layer of pure thought and energy". (VOY: "Cold Fire")

In 2375, Voyager became stuck on a subspace sandbar, which is a subspace layer in which the barrier between normal space and subspace is unstable. (VOY: "Bride of Chaotica!")

In the same year, the "telepathic pitcher plant" entity appearing as a wormhole, sensed that Seven of Nine was immune to its manipulation. It convinced the crew of the Voyager, that Starfleet on the other side had asked to put her in a stasis, because the wormhole passed through a layer of subspace monitored by the Borg and her neural transceiver might attract their attention. (VOY: "Bliss")