A tactician battle computer was a software package and transmitter that could be installed on any droid.
The tactician battle computer was paired with a transceiver on a specific ranged weapon. This prevented a droid from being able to attack using any available weapon. In fact, if the droid tried to use a non-matched weapon, the tactician battle computer would shut down completely.
In addition, the tactician battle computer allowed droids to share tactical data gathered from their sensors to other droids equipped with battle computers, increasing their accuracy.
The tactician battle computer was developed by droid manufactures to get around restrictions on 4th-degree droids.