Return on hold-- notice from IRS

I filed the very last day and have been waiting on my refund since. My husband is active duty army and was deployed last year. We have since received a notice (CP04) saying that that they need more information to verify combat tax deferment benefits. 

 

We never received one of them before. We filed on time but it still isn't accepted and now we have this notice. I have no idea what this means. Any ideas? Is this holding up our refund? Or is it a "just in case"?