About the meaning of labels in data files

[Petpurity]

Dear author, hello ! I am a graduate student majoring in statistics. I am fascinated by your article on the use of PyGSLIB for resource estimation of this geological algorithm. I have run through your code and understand the meaning of most of the data in the data file. However, the meaning of the CU data column in the assay _ BABBITT data file is not clear. Can you tell me the meaning of this data column ?