Before home theater receivers and surround sound, stereo was the primary listening option for music and movies. One feature that most stereo receivers had (and most still have) is an A/B speaker switch.
Home Theater Complete Multi Zone Zone2 How to Set Up
This switch allows a stereo receiver to connect to an additional set of speakers. These speakers are placed in the back of the room for room-filling sound or in another room for comfortable listening without setting up an additional system.
While an A/B speaker switch adds flexibility, it only allows you to listen to the same source playing in the main room. It also reduces the power going to all speakers, since the amplifiers are driving four speakers instead of two.
With the introduction of home theater receivers, which can drive five or more channels simultaneously, the idea of the A/B speaker switch was upgraded to a feature called Zone 2.
