Hi Rutger,
In response to your question, we have found that the documentation is lacking in the required information to be able to successfully use the Email Template option, and we will be reviewing this.
To assist you in the meantime, you will be able to find the fields in the Public Static Entity, ExtraField, in the CM_CaseDefinition_CS module. All of the fields available in this entity are supported by the email template and will be replaced at runtime by the CMf Send Email action.

To use these fields in your email template, all you need to do is include the field name surrounded by "{{" and "}}" and it will be automatically replaced by the specific case information. For example, to add the CaseNumber in your email's subject, you would write "Review for Request {{CaseNumber}} has expired".
You can see an example of this in the
OutTracker_CS module in the
OutTracker app, available on the
Forge.
Regarding your second question, we currently do not support custom placeholders.
Please don't hesitate to reach out again if you need more information.
Best Regards,
Paulo Sebastião