A Note (formerly called a Dev Note) is a comment type that can only be seen by the prototype owner and collaborators on a prototype. External stakeholders/clients are unable to see these comments.
We see Notes as a feature to use when handing over the prototype to your engineers. Rather than have your engineers click through all of the comments, you can leave a handful of very high level Notes specific to them. The yellow color easily distinguishes the type to ensure your engineers focus only on the notes intended for them.