The CHOICE of encoding used for the encoding of a pre-1994 EXTERNAL, as used by the Basic Encoding Rules, Canonical Encoding Rules, or Distinguished Encoding Rules.
According to the https://www.itu.int/en/pages/default.aspx, International Telecommunications Union's https://www.itu.int/rec/T-REC-X.680/en, X.680 - Abstract Syntax Notation One (ASN.1), the abstract definition for an $(MONO EXTERNAL), after removing the comments in the specification, is as follows: