There should be no wireless transmissions in the area where pyrotechnic devices are being used.
What does "wireless transmissions" mean? What's confusing for me is that whether "transmissions" refers to "transmission signal" or "transmission device". Which one is correct?
What "wireless transmissions" means is that you can use Bluetooth or WIFI to send people files. The sentence just says that you are not allowed to do that in the workplace. Transmissions refer to the act.
According to the Cambridge Dictionary, transmission means "the process of broadcasting something by radio, television, etc., or something that is broadcast". "wireless transmission" refers not to the signal or the device, but the process of transmitting.
A radio is a device that uses electromagnetic energy to send information from one place to another. The energy from a radio device (mobile phone, walkie-talkie, computer with wifi) is sent out in all directions and is strongest near the device. This energy could be picked up by any conductive material in the vicinity. If the wires to a pyrotechnic device pick up some of this energy, it could set of the pyrotechnic device: this might cause damage or injury. For the same reason, the use of mobile phones is not permitted in petrol stations and other places where there are flammable materials.
Your sentence is phrased in an unusual way, focusing on the process: it might have been more understandable expressed like this:
the use of radio transmitters such as mobile phones...
