The “mapping” or enrichment is taking place during the transformation step.
Remember: analyze (scan), organize, transform, validate (correct) and import.
Advantage: You can get up-to-date or real-time information. At any time you do the transformation, fresh data will be pulled. Extremely useful when the DEV, STAGE, PRD migration process takes a while. Also, you don’t need to maintain the data because normally it is maintained automatically by another system.
Disadvantage: You have to implement a custom function. For some customers – especially in highly regulated environments – that could be a no-go or showstopper. Because it isn’t an out-of-the-box feature, the requirements for it are unique for each project.
Additionally, and that is what you see in the screenshots, you can also get information of other objects inside the MC database. That is a real benefit if you need attributes from other documents, versions or relations. For example, you want to set the “security class” attribute but in order to do so, you might need to get the security class from its parent.
Finally yet importantly, whatever option you might choose: the beauty in migration-center is the validation step is mandatory, always. Just in case you wondered if those custom implementations are risky, at the end of the day, the validation will show any kind of errors. migration-center will never let you import unvalidated documents.