Subspace log

From Memory Alpha, the free Star Trek reference

A subspace log is a log that is automatically kept for all incoming and outgoing subspace communications.

When Ro Laren was confronted by Jean-Luc Picard about leave the USS Enterprise-D without authorization, in 2368, she told him that she had actually gotten permission via communication from Admiral Kennelly, which she said could be confirmed by the subspace log. (TNG: "Ensign Ro")

Later that year, Data and Spock, who had hacked into the Romulan Information Net, were able to access Romulan subspace logs that identified a transmission from the Romulan surface to a Barolian freighternear Galorndon Core, which simply contained a short sequence of numbers: 1400. (TNG: "Unification II")

In 2369 Constable Odo checked all of the communication logs during the Klaestron Civil War and found evidence that Curzon Dax and Enina Tandro had an affair. (DS9: "Dax")