In the first third of the detail page is a section called "Quantifying the National Debt". He goes into the components of SS, Medicare, etc..and it looks like he's trying to net the balance of funded/unfunded and using the net as a component of "social spending".
The footnotes below the chart led "Federal Government Current Expenditures by Function" contains the following footnote: † Social spending includes income security (Social Security, welfare, etc.), healthcare, education, housing, and recreation.