In Star Trek: Generations, the Duras sisters rig Geordi's visor with a camera so that they can find out the ship's "Shield Modulation". Once they have this number, they can set their torpedoes/disruptors to match it and shoot straight through the Enterprise's shields.
Is it ever established what this number means? Why have a specific frequency or whatever that allows people to shoot through your shields?
The best I can come up with is that you need to set your own torpedoes to this frequency so you can shoot out through your own shields, but is this backed up by anything? It also seems strange they'd display such a number - which is the equivalent of a password to your ship - so prominently on the screen. Also, if this were the case, then surely if the Duras' changed their weapons to match the Enterprise's shield modulation, they wouldn't be able to shoot through their own shields.