Which service allows a data engineer to look up all metadata for a given batch ID via API?

A data engineer ingests 1000 records that contain various different identities. Each record has at least the primary identity.
The data engineer verifies that the records have been ingested into Data Lake and profile. When clicking on one of the identity namespaces in the identity tab the data engineer sees 100 records under "Records skipped".
What is the possible cause of the skipped records?

Which service allows a data engineer to look up all metadata for a given batch ID via API?

A data engineer builds a segment based on Loyalty Status = Gold attribute and a purchase in the last 7 days.
To validate that this segment is working, the engineer logs in to the test website and makes a purchase Gold Loyalty Status.
In AEP. how can the data engineer validate that the test customerlD made it into the segment near real-time?

You want to load purchase order data into The Adobe Experience Platform You know that every event loaded into the Adobe Experience Platform requires a unique ID (_jd). Given the sample record below:
Which field or fields can be used for this ID?