A plasma flare was a relatively harmless projectile device.
Plasma flares are one of the components in a biogenic weapon delivery system. In 2370, Starfleet planted disinformation amongst the Maquis suggesting that the Cardassians were assembling such a weapon in the Demilitarized Zone. The last components of the weapon were to be isomiotic hyposprays, quarantine pods, and plasma flares, being carried on a Yridian freighter. (TNG: "Preemptive Strike")
In 2376, a band of con artists led by Dala, pretending they were from the USS Voyager, sold Varn some plasma flares as "photon torpedoes". When Varn confronted them with the fact, Dala suggested that maybe he "didn't arm them properly". (VOY: "Live Fast and Prosper")