とりあえず雑に見つけたドキュメントだとこんな感じ

/// A mapping from each requirement of a protocol to the SIL-level entity /// satisfying the requirement for a concrete type. class SILWitnessTable : public llvm::ilist_node<SILWitnessTable>, public SILAllocated<SILWitnessTable>

/// A mapping from each requirement of a protocol to the SIL-level entity /// satisfying the requirement for conformances which do not explicitly /// provide a witness. class SILDefaultWitnessTable : public llvm::ilist_node<SILDefaultWitnessTable>, public SILAllocated<SILDefaultWitnessTable>