How does a schema change in forest A affect the schema of domains in forest B?

In the context of Active Directory, when a schema change occurs in one forest, it does not automatically propagate or affect the schemas of other forests. This principle is fundamental to how Active Directory forests are structured. Each forest maintains its own independent schema, meaning that changes made in forest A's schema won't have any impact on the schema of forest B.

Therefore, if a schema modification—such as adding new object classes or attributes—takes place in forest A, it is entirely confined to that forest. Forest B's schema remains unchanged and unaffected by any alterations made in another forest. This independence allows for flexibility in managing different organizational needs across separate forests without the risk of unintended consequences from schema alterations in another forest.

This is why the assertion that changes do not affect forest B is accurate.