It is a catch all that includes anything not related to audio. One of the types of electronics I have designed was test equipment that tested audio circuitry (at Altec Lansing). This type of circuitry obviously must be a stable as possible, yet it uses the many of the same types of circuits and parts as the audio equipment in the home. Knowing what types of parts and circuits are subject to changes and which aren't is very important in this types of design, and knowing what to do about it.