EWC code
European Waste Catalogue, the List of Waste
The short answer
The six-digit code that classifies every waste stream, grouped into twenty chapters by the industry or process that produced it. An asterisk means hazardous.
In practice
The code goes on the transfer note and into the DEFRA filing, so it is the field most likely to be wrong and least likely to be questioned. The chapter is chosen by where the waste came from, not by what it looks like, which is why the same material can sit under different codes for different producers.