It is very difficult to give examples on this point, since it is for every organization different on how to implement this part.
The idea behind the Data registry is that a DPO can set purposes (why the organisation is processing data) with retention periods en categories for data stored in Moodle in the data registry.
By example you make a category Identifcation data with the purpose student administration.
Please bare in mind - this is an example - which does not necessary counts for your organization.
If you get stuck on this part, the only and best advice someone can give you is to contact yoru organizations IT depertmant or specialist on GDPR.
For this example see: https://docs.moodle.org/34/en/Data_privacy_plugin
And you might already have read: https://docs.moodle.org/34/en/GDPR_for_administrators