Where and how is occurrence calculated?

[bondijoe27]

reports = read_indexer_reports(report_df, entity_df, COMMUNITY_LEVEL)
I follow global serach example notebook, then I found after this step, for elements in this reports list, it has attributes called 'occurrence'. I know it could be used later at global search step. I want to know how is it calculated.
After I check indexer_adapters function and read_community_reports function, attributes_cols is None not ['occurrence'], which is my question here.