It is the line number for the individual as listed in a separate 1930
census of unemployment. See references below:

A census of Unemployment was conducted, in conjunction with the 1930
census, by an act of May 3, 1928. This special enumeration collected
data on persons who usually worked for wages or a salary, but were not
working at the time the census was taken.
Enumerators were instructed to complete an unemployment schedule for
every person responding ?No? in column 25 of the general population
schedule. The ?unemployed? were grouped into two classes?those having
a job but temporarily laid-off on account of a lack of orders,
weather, sickness, etc.; and those who were unemployed but want to
work.
The unemployment census provided data concerning the number of men and
women unemployed the average age of the unemployed, how many of the
unemployed were married and single, how long they had been out of
work, and the leading reasons for unemployment in the United States.
Data were made available for the Nation, individual segments of the
population (i.e., by age, race, marital status, etc.), and for the
foreign-born and native populations.From:
http://www.census.gov/prod/www/abs/decennial/1930.htm#three

See Appendix A for further information about the special unemployment
census: http://www2.census.gov/prod2/decennial/documents/03453339v2_TOC.pdf

mpalmer@... wrote in part:

In the 1930 census, my grandfather is listed as not "actually at work
yesterday," with a 5 as the code on the "unemployment schedule." Does
anyone know what the 5 stood for (other codes on the page ranged >from 6-21)?
Thank you.
Michele Winitsky Palmer