User IDs in the API

Hi folks, I'm building an API integration to sync Maximizer data with another CRM. We need an ID that can be used as a unique identifier to match records with. In the Maximizer Ferret API results I see the "legacyKey" value, but I don't see this value anywhere in the Maximizer CRM user interface. In the interface I see the "IDentification" under System Information, but I don't see number anywhere in the API data.

So my question is, what ID can I use as a unique identifier for records in Maximizer, which is available in both the API and the CRM user interface?

Thank you!