8.1What should be configuration?

Suppose your Mule code talks to:

Salesforce
Billing API
Database
Kafka

Things that differ by environment include:

Salesforce endpoint
OAuth client ID
OAuth secret / certificate
HTTP host
HTTP port
database URL
queue names
timeouts
retry counts
feature flags
logging levels

You do not want:

<http:request host="qa.billing.mycompany.com"/>

hardcoded.

Instead:

<http:request host="${billing.host}"/>

Then configuration supplies the value.