XCAR stands for "Extended Customer Analytical Record" and refers to the concept of a single object, centered around the Customer (or other entity your decision strategies reason about), extended with data from various sources (e.g. social, marketing insights, real time activities etc) that you could possibly want to use in your decision strategies.
In summary: a data "record" that combines all customer related sources.
The actual data model for the XCAR differs from customer to customer. The Decision Data Set allows for easy storage of XCAR (or any object) in a Cassandra based big data store.
I suggest you talk to your client to make sure they're familiar with concepts like Data Sets, Data Flows and how these are becoming increasingly important in recent product releases.