A weapon signature was a specific energy reading that could be used to identify which species' technology produced a particle discharge. This could prove very useful in determining who was responsible for an attack.
The Romulan drone-ship's disruptors utilized tri-phasic emitters, which allowed them to imitate the weapon signatures of various other species, including Tellarite disruptors, Andorian particle cannons, and Starfleet phase cannons. (ENT: "Babel One")
In 2154, Lieutenant Malcolm Reed identified a Klingon weapon signature when investigating the destruction of a Rigelian freighter, although he lied to Captain Archer, claiming that the weapon signature was actually Orion, under orders from Harris, his former commander in Section 31. (ENT: "Affliction")