[from telecommunications and network theory]
1. In software, describes values of a function which are not in its ‘natural’ range of return values, but are rather signals that some kind of exception has occurred. Many C functions, for example, return a nonnegative integral value, but indicate failure with an out-of-band return value of −1. Compare hidden flag, green bytes, fence.
2. Also sometimes used to describe what communications people call shift characters, such as the ESC that leads control sequences for many terminals, or the level shift indicators in the old 5-bit Baudot codes.
3. In personal communication, using methods other than email, such as telephones or snail-mail.