SOS
is possibly used for encoding sub-elements or accessing metadata which is required for the encoding, like time stamp format.
signature(obj = "OgcBinaryTemporalOp", sos = "SOS")
signature(obj = "SosEventTime", sos = "ANY")
signature(obj = "POSIXt", sos = "ANY")
SosBindings
, http://en.wikipedia.org/wiki/Key_Value_Pair