What do the field names (e.g., AG3001, B5P001) in an NHGIS data file mean?

What do the field names (e.g., AG3001, B5P001) in an NHGIS data file mean?

The key to these unique, NHGIS-created column names is found in the Codebook file(s) that were automatically included in your data extract. Look for the .txt file(s) in the zipped file you downloaded, and they will shed some light on your data.