On some test cards and Test Transmissions, they say that the transmitter is on Reduced Power. Is it the obvious meaning of that they are using reduced power to send the signal or does it mean something else?
Reduced Power means that the transmitter is outputting at a lower power than usual and therefore reception might be worse, especially for those on the edge of its area.
Main, high powered transmitters are actually in pairs. There are 2 transmitters running simultaneously and their outputs are combined before being sent up to the aerials. This is done to provide redundancy, if one of the transmitters fails or needs to be switched off to be maintained the other one will continue to provide transmissions, albeit at a total lower (reduced) power.
Reduced power might also be caused by the transmitter site using a reserve transmitter (which wouldn't normally be as powerful as the main one), or reserve aerials. These sometimes can't take the power that the main ones can and in most cases are lower on the mast so don't cover as big an area.
Reserve aerials and reduced power working are normally used when work is taking place on the mast, it's dangerous to the workers to come into contact with the very high levels of RF coming from the aerials so it's either reduced, taken off air completely or switched to aerials away from where the work is taking place