The essential difference between Code and Convention is that the former becomes Mandatory for Contracting Parties when it enters into force. Convention is an agreed set of rules on a particular matter. The need for Convention is mostly triggered by a Major Incident (eg Titanic) or a result of a wide spread research (eg. Ballast water Convention). A code on the other hand is normally recommendatory unless it is made mandatory under the provision of the convention.
So a Code to a convention is part of the convention. It provides the international standards required for the elements mentioned in that particular chapter of the convention.