The best source for clarification on these variables would be the enumeration forms, and the detailed instructions to respondents, both of which can been found on the IPUMS-USA website. The IPUMS-USA income variables you are asking about directly correspond to single question items on the ACS form (for 2010 these are questions 47a - 48).
To answer your specific questions, because INCRETIR specifically excludes Social Security, one would assume that INCSS only includes Social Security (and railroad retirement) benefits. The detailed instructions relating to INCSS note, “Include amounts, before Medicare deductions, of Social Security and/or Railroad Retirement payments to retired persons, to dependents of deceased insured workers, and to disabled workers.”
Also in the detailed instructions, respondents were instructed not to include “Capital gains or losses from the sale of homes, shares of stock, etc.” under any item.
Workers’ comp is never directly mentioned in the survey, however, as noted above, benefits to disabled workers are counted under INCSS.
Unemployment and child support are both included in the INCOTHER variable. From the detailed instructions: “Include Veterans’ (VA) disability compensation and educational assistance payments (VEAP); unemployment compensation, child support or alimony; and all other regular payments such as Armed Forces transfer payments, assistance from private charities, regular contributions from persons not living in the household.”
I hope this helps.