There are a few points that confuse me about the credentials we define while activating SASL/PLAIN. I've added these lines to server.properties.

sasl.enabled.mechanisms=PLAIN
sasl.mechanism.inter.broker.protocol=PLAIN
security.inter.broker.protocol=SASL_PLAINTEXT

listener.name.sasl_plaintext.plain.sasl.jaas.config=org.apache.kafka.common.security.plain.PlainLoginModule required \
   username="admin" \
   password="secret" \
   user_admin="secret";

And added these lines to Schema Registry and Connect to make them be able to communicate with broker.

sasl.mechanism=PLAIN
security.protocol=SASL_PLAINTEXT
sasl.jaas.config=org.apache.kafka.common.security.plain.PlainLoginModule required \
  username="admin" \
  password="secret";

So, what is the purpose of user_admin in server.properties? If it is the one we use for other components to connect, what does the username and password fields do? Is it only used for inter-broker communication? Is there any relation between user_admin and the admin defined above?

I couldn't find a clear explanation, thanks in advance.