There is a simple explanation in the Digest Emails.

1. Enabled Debug Mode

2. From the Top Nav bar in the right Select 'Technical > Email > Digest Email'

3. Press 'CREATE'

4. in the New Tab press 'How to Customize your digest?, Here below is what reports.

5. 
   

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In order to build your customized digest, follow these steps:

1. You may want to add new computed fields with Odoo Studio:
   • you must create 2 fields on the digest object:
   • first create a boolean field called kpi_myfield and display it in the KPI's tab;
   • then create a computed field called kpi_myfield_value that will compute your customized KPI.
2. Select your KPIs in the KPI's tab.
3. Create or edit the mail template: you may get computed KPI's value using these fields:

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The Digest Module is found in 'odoo > Addons > Digest'.

From that Folder you can find a template: 'Views > digest_templates.xml'

From 'Models > digest.py' you will find function:

With the following dictionary: