What is KMS Key Material?

Key material is the string of bits used in a cryptographic algorithm. Secret key material must be kept secret to protect the cryptographic operations that use it. Public key material is designed to be shared.

Each KMS key includes a reference to its key material in its metadata. The key material origin of symmetric encryption KMS keys can vary. You can use key material that AWS KMS generates, key material that is generated in the AWS CloudHSM cluster of a custom key store, or import your own key material. If you use AWS KMS key material for your symmetric encryption KMS key, you can enable automatic rotation of your key material.

By default, each KMS key has unique key material. However, you can create a set of multi-Region keys with the same key material.

https://docs.aws.amazon.com/kms/latest/developerguide/concepts.html#key-material

Documentation on how to import Key Material into KMS: https://docs.aws.amazon.com/kms/latest/developerguide/importing-keys.html