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How ASN.1 builds new terms out of existing terms

ASN.1 defines each term as a sequence of components, some of which may be sequences themselves. To give a simplified example, here's how you might describe a letter in ASN.1:

Letter ::= SEQUENCE {
      opening       OCTET STRING,
      body           OCTET STRING,
      closing        OCTET STRING,
      address       AddressType
}

Note that while most of the elements in this sequence are defined using a primitive element (the "octet string," which is the equivalent of a byte), the address is simply defined as a text string, "AddressType." You can do this because AddressType is defined in another sequence, like so:

AddressType ::= SEQUENCE {
      name      OCTET STRING,
      number    INTEGER,
      street      OCTET STRING,
      city          OCTET STRING,
      state       OCTET STRING,
      zipCode   INTEGER
}

For a computer parsing the sequence "Letter," AddressType will be read as an instruction to insert the octet string and integer structures listed in the sequence that defines AddressType.

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