Signal operating instructions
(SOI) is a U.S. military term for a type of combat order issued for the technical control and coordination of communications within a command. They include current and up-to-date information covering radio call signs and frequencies, a telephone directory, code-words (for rudimentary encryption), and visual and sound signals. A designated battalion
signal officer prepares the battalion SOI in conformance with the SOI of higher headquarters.
Units maintained 2 copies of the SOI: a training version and a "go-to-war" version. Since the fielding of the SINCGARS
system, however, the paper SOI has generally faded from Army use. Electronic SOI are now generated, distributed and loaded along with cryptographic keys