The last optional waveform is sample, which is only enabled when you have a .wav file in your project with the same name as a sound effect. For example, if you export the first default sound, which is named Melody, and copy the .wav file to the root of your project folder like so, it will enable the sample option for this sound effect.
When you open the Sound Tool, the Melody sound effect will be set to Sample. In this mode, you are only able to preview the sound effect and change its volume. All of the dynamic sound effect options will be disabled.
This is a powerful option and allows you to simulate how the original NES had a dedicated sample channel.