Retention Labels and destroying records

To make sure we're talking about the same thing...

You can create a classification label that is applied automatically or manually to content (email and documents). The label can mark classified items as permanent records (see below).

Label marking item as a record

You can then publish the labels in policies to make them available to users or as auto-label policies to have them applied automatically to content that satisfies certain criteria.

When an item is marked as a permanent record, it is kept for the retention period specified by the label and then the retention action is executed (in the case above, the item is deleted after 20 years). No other action can happen to it during that time. From https://support.office.com/en-us/article/Overview-of-labels-af398293-c69d-465e-a249-d74561552d30: 

Implement records management across Office 365, including both email and documents. You can use a label to classify content as a record. When this happens, the label can’t be changed or removed, and the content can’t be edited or deleted.