Economic Census Data Dictionary: those footnotes
From the US Census Bureau A repository of information describing database attributes. The dictionary contains an identification section to describe record codes, geographic area codes, area characteristics, and special area codes, plus endnotes and a table (matrix) section. Disclosure In accordance with federal law governing census reports (Title 13 of the United States Code), no data are published that would disclose the operations of an individual establishment or business. In cases where the data must be withheld, the following symbol appears in place of the data: D : Withheld to avoid disclosing data for individual companies; data are included in higher level totals Employment Ranges Used when the sales and other data must be withheld due to disclosure, but a range for the employment data can be shown. The following symbols are valid: a : 0 to 19 employees b : 20 to 99 employees c : 100 to 249 employees e : 250 to 499 employees f : 500 to 999 employees g : 1,000