Master key
A master key can technically be used to sign without a need for a subkey. However, we recommend that you only use the master key (sometimes called “certification key”) to certify and create subkeys.
A GPG master key contains:
RSA, ECDSA, or EdDSA keypair.
User IDs (UIDs).
Self-signature for every UID associated with the master key.
Key that can certify.
The master key can be used to:
Add or revoke subkeys.
Add, change, or revoke user identities (UIDs) associated with the key.
Add or change the expiration date on itself or any subkey.
Sign other people's keys for web-of-trust purposes.