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Indicated by type="Chaining", allows multiple sources of credentials to be queried in sequence.
With V2.4 and above, this is implied by any configuration with multiple <CredentialResolver> elements, so is no longer explicitly needed.
Primarily used to allow different keys and certificates to be used based on the identity provider. While this works in some situations, it can be complex to get working, and it's strongly advisable for SPs to maintain a single set of credentials to use for all interactions.
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