Receiver groups are an alternative way to define who receives an event. It's an option available with operation RaiseEvent.
Usually, Photon LoadBalancing sends your events to everyone else in the same room. With the receiver group parameter for RaiseEvent, you can send an event to "All", including yourself. If needed, you can also send to only the MasterClient.
This is an alternative to listing all target players, which is still possible in RaiseEvent.