This is question that was described in detail in Chapter 3 of the Journey towards a digital transformation in the process industries (Draft 2017).
In a nut shell, the PI Events Frame Template collects all the events for the PLANT units tracking the performance of every process unit.
The production rates for each unit, the production variances (Modes) and the key consumable variable data are captured by the PI Event Subsystem.
The PI AF generated table is exported to PowerBI for Analysis and Visualization.
Here is where you can get answers to questions about the data from Cortana. It is based on the Assets (Units), Production Operational Modes and the process variables chosen to configure the Event Frame Template.
Attached is a presentation with the ways to teach the computer to answer Plant questions. It expands into how to train the computer to do predictive analystics for the Selected operating Modes.