Every single element, in every single mailing, has a tick box in the Visual Editor that says 'use content from xxx'. 

If the box is ticked, the version is inheriting from the mailing or template specified. 

If the box is unticked, then the version is not inheriting. If you change something in a version that is not the master, that automatically breaks the inheritance.   

Useful Links:
What is inheritance? | Restoring Inheritance | Working with Inheritance | Master Versions vs Non Master Versions | Breaking Inheritance

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