Letters of Volatility document the volatile and non-volatile memory locations in a hardware device. This information is necessary for protecting devices when they are removed for repair, reassignment, or disposal to ensure that data is not accidently made available to other people.
NI Letters of volatility include the following sections:
The memory lists include whether the location has a battery backup, is user accessible, and is system accessible. The lists also include a description of the clearing procedure for each memory location. Clearing instructions are documented as procedures. Procedures may be physical actions like unplugging a backup battery. A procedure may be a series of function calls that can be used to erase the data.
Notice that there are different levels of sanitization of these locations. NIST defines sanitization in the document Guidelines for Media Sanitization, NIST 800-88. NI's Letters of Volatility offer a method of clearing data. Some devices may require more advanced sanitization techniques or tools.