one declared member of a SigClassInfo: an S4 slot, an S7 property, an RC field, or an R6 entry
Readonly
Optional
the class the declaration states for the member ("numeric", class_numeric); absent when it states none
"numeric"
class_numeric
one declared member of a SigClassInfo: an S4 slot, an S7 property, an RC field, or an R6 entry