A capital S in the margin of an entry is an abbreviation for Latin word soca, translated as jurisdiction in this edition but more commonly as soke or sokeland.
A soke was an outlying dependency of a manor which rendered customary payments to the lord of the manor but whose soil was not owned by him. In circuit 6, where all holdings were classified as manors, jurisdictions or outliers, sokelands were emphasised by this marginal S.
For the manorial structure of circuit 6, see F.M. Stenton, Types of manorial structure in the northern Danelaw (1910); and Dawn M. Hadley, The northern Danelaw: its social structure, c.800-1100 (2000).