A routine ID is an ID used to identify a set of features which are redeclarations of a unique ancestor. This is used for dynamic binding.
Every feature has a routine ID set. The set is most of the time made of one element because most feature are the declaration themself, or the redeclaration of just one ancestor. When you have more than one element, it means that it is the redeclaration of more than one ancestor (e.g. case of merging of two features coming from two different parents).