Computing

• Digraphs and trigraphs, groups of characters used to symbolise one character
• An octal or decimal representation of byte
  Byte
  The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
  
   values.
• Mnemonics for machine language instructions.
• As language codes in ISO 639
  ISO 639
  ISO 639 is a set of standards by the International Organization for Standardization that is concerned with representation of names for language and language groups....
  
  

Cryptography

• As substitution group in a substitution cipher
  Substitution cipher
  In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the "units" may be single letters , pairs of letters, triplets of letters, mixtures of the above, and so forth...
  
  
• As combinations in the Ling Qi Jing

Mathematics

• As a generalization of graph
  Graph (mathematics)
  In mathematics, a graph is an abstract representation of a set of objects where some pairs of the objects are connected by links. The interconnected objects are represented by mathematical abstractions called vertices, and the links that connect some pairs of vertices are called edges...
  
  s where we have a set of edges called semi-adjacent