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

Battalion

A battalion is a military unit of around 300–1,200 soldiers usually consisting of between two and seven companies and typically commanded by either a Lieutenant Colonel or a Colonel...

 signal officer prepares the battalion SOI in conformance with the SOI of higher headquarters. http://www.globalsecurity.org/military/library/policy/army/fm/55-30/apps.htm Units maintained 2 copies of the SOI: a training version and a "go-to-war" version. Since the fielding of the SINCGARS

SINCGARS

SINCGARS is a Combat Net Radio currently used by U.S. and allied military forces. The radios, which handle voice and data communications, are designed to be reliable, secure and easily maintained...

 system, however, the paper SOI has generally faded from Army use. Electronic SOI are now generated, distributed and loaded along with cryptographic keys

Key (cryptography)

In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...

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