SubscriptionSupport

The names of the product elements

The name of the type

The depth a subscription actually waits, which is not always what it asked for: noRollback means "wait until a reorg cannot reach me", i.e. the provider's declared rollback horizon. The hub computes delivery depth the same way, and both must, or a subscription can be accepted against maxConfirmations and then gated far deeper than the provider can manage.

Classify a request against a provider's declared capabilities.

This is the single implementation used by callers deciding what to ask for, by every provider's own subscribe-time validation, and by the conformance suite — so the rule "subscribe throws exactly when this returns Unsupported, or returns Unindexed while allowUnindexedScan is false" holds by construction rather than by convention.

It is also the one place composite queries are reasoned about: a union is indexed only if every arm is, while an intersection needs just one indexed arm because the rest post-filters. That rule replicated across four providers would be four chances to get it subtly wrong.