Once you create a Reference model you can then enable the GDPR analysis functionality to evaluate sensitivity of your captured data.
Enable the ref model
Select the ref model and switch the toggle "GDPR enabled" in the "DETAILS" tab.
Enable each node
You can make any node, at any level, "GDPR enabled". Toggle the switch in the GDPR tab.
For Salesforce Ref model this is set automatically at field level.
Enable all children nodes
You can also enable multiple nodes at the same time. Select a node, and on the right panel toggle the second switch in the GDPR tab.