Perhaps someone more familiar with such actions than I am can explain the significance of this in terms of what action is then required by Barr. The decision to prosecute or not is his, but is he then obligated to explain why he chooses not to, if that is the decision made?
“The decision to prosecute or not is his, but is he then obligated to explain why”
As a practical matter yes. But, as long as the investigation (or a related investigation) is ongoing, he should NOT provide any details.
So an AG who seeks to stonewall or slow roll Congress (like Eric Holder) could hide from explaining why for years.
But in an honestly functioning Government, yes, the AG would be obligated to explain his decision eventually. Devin Nunes is on the Congressional Committee with direct oversight of DoJ.
A criminal referral or criminal recommendation is a notice to a prosecutory body, recommending criminal investigation or prosecution of one or more entities for crimes which fall into that body’s jurisdiction. ... In a direct referral, agencies refer cases to the U.S. Attorney in the district where the crime occurred.
Criminal referral - Wikipedia
https://en.wikipedia.org/wiki/Criminal_referral