My understanding is that your client's identity may have been compromised in the last year, either attempted refund theft or some other manner. The IRS will mail a letter with the PIN which I believe is a unique 5 digit pin that you must get from the client's IRS letter, and insert in that blank field. The pin will change each year until the IRS determines that they no longer have a concern with the client's identity. I have several clients who get these letters and don't know what to do and unfortunately may toss them. Good luck