The Safety Release Valve Note is a mechanism, created by Kenneth Folk, by which one always has a note that they can use, even when you don't know what is happening in your experience, or you're not sure how to use it. The way it works is when you don't know what or how to note you can always note, "Don't Know", "Uncertain", "Pass", "Thank You" (particularly good for Social Metta practices) or any variation on that theme.
The Safety Release Valve Note was designed for Social Noting, to help reduce social anxiety that can build up around not being sure what to note (especially for beginners) and to keep the flow of the noting practice unimpeded by excessive deliberation. It's especially useful to employ when working with a technique that has a limited range of options, in terms of what to note (ex. Six Sense Noting), but can be used more widely.