These symmetrical measures are appropriate when no partial similarities can be derived from variable values, that is:
objects are either in a category or outside of it, where categories are equally (dis)similar.
objects either have an instance of a variable (e.g. presence of a species) or lack it.
The "Sn" notation was adopted by Legendre and Legendre (1998) and is reproduced below to assist cross-referencing.
Other coefficients derived from those above are noted by Legendre and Legendre (1998). As always, consider what kind of distances are suitable for your questions and analysis prior to selecting a measure or searching further.
Legendre P, Legendre L. Numerical Ecology. 2nd ed. Amsterdam: Elsevier, 1998. ISBN 978-0444892508.
Sokal RR, Sneath PHE. Principles of Numerical Taxonomy. San Francisco: Freeman & Co., 1963.