What does the coding N/A mean in the variable IND1950?

The variable IND1950 includes a lot of 0 or N/A, which means that they are not reported industries. I was wondering if there is a more detailed description/information of N/A and what kind of industries that could be in N/A?

A fast reply would be much appreciated. Thanks in advance!

Those respondents that do not meet the criteria described in the IND1950 Universe statement are coded as IND1950=“000:N/A or none reported”. In general, this means the respondent was either too young, had yet to secure a first job, or had not been employed for several years. In the 1850-1930 samples, persons of all ages were included in the Universe; however, if the respondent did not report an occupation, then industry was left blank (i.e. “none reported”).

Hope this helps.