The main distinction to make is between the carrier (or medium) and the content. A CD or cassette tape is a physical carrier. The audio content is the sound recording or signal embedded into the carrier. A CD has a digital signal recorded as pits or indentations in the CD, while a cassette tape is commonly an analogue signal embedded as a signal in the magnetic coating of a tape (although magnetic tape can also be used to record digital signals if required).

The advent of cheap computer hard drives and flash storage, including those in digital audio players, means it is more common to have a digital signal that can be stored and copied to multiple carriers without degradation or loss of quality, depending on how the copying is done.

Many libraries and archives historically collected records of every different medium for an audio recording. With the decline of the prevalence of discrete physical media (such as vinyl LPs, tapes and CDs), a number of these collecting institutions now only collect one copy of the recording in a form that is most readily available or most accessible for their customers.

In most cases then, the record classification relates the content, with the carrier or medium it is stored in being a secondary consideration.

If you want to know more about digital audio recording, check out our digital audio guide http://makeit.digitalnz.org/guidelines/creating-digital-content/creating-digital-audio/