Field Location and API Name:
* The custom text field Additional_Text__c resides on the Quote object.
* In Salesforce CPQ, the correct syntax to reference fields in HTML templates is to use the merge field syntax appropriate to the object.
Syntax Explanation:
* {!quote.Additional_Text__c} directly references the field from the Quote object in a format recognized by the CPQ document generation engine.
* Alternative options such as {ISBQQ__Quote__r.Additional_Text__c} reference lookup relationships and do not apply here.
Salesforce CPQ Reference:
* As per the Quote Template Content documentation, use the merge fields directly associated with the object being utilized in the template .