Hi @koryclick,
Thanks for your question.
Target Class is an annotation we get from ChEMBL’s protein class data (the classification that you see in a ChEMBL target report card in the ‘Protein Target Classification’ row).
Their publication describes the classification: protein targets are classified into a manually curated family hierarchy, according to nomenclature commonly used by drug discovery scientists (not aiming to be comprehensive).
Finally, it’s a known issue that targetclass_id has been missing since ChEMBL32. We’ve raised it with ChEMBL.
Let us know if you have further questions!