You need both, and each one has a specific role.
***ca.p12 is the certificate to the new Certificate Autohirity which is created since Elastic certificated is auto assigned.
***certificate.p12 is the certificate to each single instance of your elasticsearch cluster.
I would recommend you to follow the steps here https://www.elastic.co/guide/en/elasticsearch/reference/current/security-basic-setup.html
Source:
https://www.elastic.co/guide/en/elasticsearch/reference/current/certutil.html#certutil-ca
ca.p12 -> CA Mode
The ca mode generates a new certificate authority (CA). By default, it produces a single PKCS#12 output file, which holds the CA certificate and the private key for the CA. If you specify the --pem parameter, the command generates a zip file, which contains the certificate and private key in PEM format.
certificates.p12 -> Cert Mode
The cert mode generates X.509 certificates and private keys. By default, it produces a single certificate and key for use on a single instance.
To generate certificates and keys for multiple instances, specify the --multiple parameter, which prompts you for details about each instance. Alternatively, you can use the --in parameter to specify a YAML file that contains details about the instances.