Content-id referencing uses the magic character $ followed by a token to refer back to a request that specifies that token in its content-id header.

Using one of the standard resource names without a $ prefix as a content-id could lead to confusion as URLs relative to the batch request URL are also relative to the service root and the relative URL for the standard resources thus starts with a $:

--batch_36522ad7-fc75-4b56-8c71-56071383e77b
Content-Type: application/http
Content-Transfer-Encoding:binary
Content-ID: batch

POST Customers HTTP/1.1

<new customer body>

--batch_36522ad7-fc75-4b56-8c71-56071383e77b
Content-Type: application/http
Content-Transfer-Encoding:binary
Content-ID: nested-batch

POST $batch HTTP/1.1
Content-Type: multipart/mixed;boundary=nested_batch_77162fcd-b8da-41ac-a9f8-9357efbbd
Content-Length: ###

--nested_batch_77162fcd-b8da-41ac-a9f8-9357efbbd

<content of nested batch

-nested_batch_77162fcd-b8da-41ac-a9f8-9357efbbd-

-batch_36522ad7-fc75-4b56-8c71-56071383e77b-

In this (admittedly weird) example the second POST request URL would not be interpreted as a content-id reference but literally as a request to $batch.