The Financial Information eXchange tagvalue encoding is the original encoding used for FIX messages. The tagvalue encoding is the encoding used by the FIX session layer; it corresponds to the Presentation Layer of the ISO Open Systems Interconnection model. The encoding uses an integer number known as a tag to identify the field, followed by the “=” character (hexadecimal 0x3D), then the value of that field encoded in the ISO 8859-1 character set. Each tagvalue pair is separated by the Start of Heading control character <SOH> (hexadecimal value 0x01), which is defined by ISO 6429:1992. The tagvalue encoding also supports the encoding of binary and multibyte character data in certain encoded data fields that are preceded by a Length field.
FIX TagValue Encoding V1.0 – Technical Standard