"The “S” in Type S hydrated lime, as defined by ASTM C 206 and ASTM C 207, was coined because
the hydrated lime was considered to be “Special”. Its special qualities include high early plasticity,
high water-retentivity values, limited oxide content, and minimal coarse fraction. These properties
enhance the performance of both mortar and plaster. The same descriptor and emphasis on plasticity
and water retentivity do not occur in any other national standard hydrated lime specification."
Same as type S cement, I guess. Not food grade in any case.