An NDA, short for "Non-Disclosure Agreement", is a form of Signamancy Contract used to prevent a signee from sharing information with others. A signee under the effect of an NDA is prevented from breaking it by magical means whenever they consider or come close to breaking it. Charlie frequently uses these as part of his arrangements to hide the nature of his involvement, and protect his own secrets.
A Pinky swear not to share a secret can have a similar effect, such as when Parson pinky promised the Great Minds not to speak of their secrets. It is unknown if such a pinky promise is somehow less powerful or binding.