From the Existential Metadata section of the ABI Stability manifesto:
Existential type metadata contains the number of witness tables present, whether the type is class-constrained, and a protocol descriptor for each protocol constraint. A protocol descriptor describes an individual protocol constraint, such as whether it is class-constrained, the size of conforming witness tables, and protocol descriptors for any protocols it refines. Protocol descriptors are layout compatible with the Objective-C runtime's protocol records on Apple platforms. The format of the existential type metadata needs to be reviewed as part of the ABI definition.