How to analyze AJO data in CJA

Responses below 

1. Can we use a custom dataset instead of a system dataset? If yes, how can we ingest AJO data into that?

>> You can use custom datasets. If you are referring to some additional data sources which you want to bring in for analysis purpose, just ensure the person Id matches. Second part of your question is covered as part of #3, I believe.

As long as the person ID matches between datasets, Customer Journey Analytics can connect filters, attribution, flow, fallout, and so on across datasets.

2. If we are using a system dataset, what will be the Person ID that we need to use while creating the connection?

>> you will have to pick it up from identityMap (say, for example if you are referring to message feedback dataset)

Reference - https://experienceleague.adobe.com/docs/analytics-platform/using/cja-connections/create-connection.html?lang=en#id-map

3. In a system dataset, will data be automatically ingested, or do we need to do anything?

>> System datasets will have data flow into it when there is a corresponding activity recorded  by the profile (For eg:- email sending would generate data against message feedback dataset)