Most earnings used to compute a Social Security benefit are "regular" earnings; that is, employer/employee (FICA) or self-employment (SECA) taxes were paid on them. However, there are two other types of earnings or credits which may be added to regular earnings to compute a benefit: 1) railroad earnings, and 2) military service wage credits.
It is also possible that a worker would have no regular earnings, and would have only railroad earnings and/or military service wage credits. In that case, check this box. Note that if you check this box, you must also check one of the next two boxes: railroad earnings and military service.