The ASN.1 EXTERNAL type is a useful type used to include non-ASN.1 or other data within an ASN.1 encoded message. This type is described using the following ASN.1 SEQUENCE:

   EXTERNAL ::= [UNIVERSAL 8] IMPLICIT SEQUENCE {
      direct-reference OBJECT IDENTIFIER OPTIONAL,
      indirect-reference INTEGER OPTIONAL,
      data-value-descriptor ObjectDescriptor OPTIONAL,
      encoding CHOICE {
         single-ASN1-type [0] ABSTRACT-SYNTAX.&Type,
         octet-aligned [1] IMPLICIT OCTET STRING,
         arbitrary [2] IMPLICIT BIT STRING
      }
   }

The ASN.1 EXTERNAL type is mapped to a Go struct, generated from the above SEQUENCE definition.