Fromhttps://docs.aws.amazon.com/kms/latest/developerguide/custom-key-store-overview.html

For most users, the default AWS KMS key store, which is protected by FIPS 140-2 validated cryptographic modules, fulfills their security requirements. There is no need to add an extra layer of maintenance responsibility or a dependency on an additional service. However, you might consider creating a custom key store if your organization has any of the following requirements: Key material cannot be stored in a shared environment. Key material must be subject to a secondary, independent audit path. The HSMs that generate and store key material must be certified at FIPS 140-2 Level 3.https://docs.aws.amazon.com/kms/latest/developerguide/custom-key-store-overview.html

https://docs.aws.amazon.com/kms/latest/developerguide/multi-region-keys-overview.html