4.11External ID

Do not confuse:

Salesforce Id

with:

External ID

Salesforce ID:

001K000001ABCDEF

belongs to Salesforce.

External ID might be:

CUST-10042

belonging to your source system.

Example:

ERP

customerId
CUST-10042

       ↓ Mule

Salesforce

Id:
001A...

External_Customer_ID__c:
CUST-10042

Why is that valuable?

Because Mule doesn't need to somehow remember:

CUST-10042 → 001ABC

in its own database just to identify the Account later.

Salesforce itself can maintain that relationship.