The IRS uses Letter 3064C for to inform you of decisions they have made or information they need. If it changes your tax amount the circumstances should be investigated. Plus, there may be implications on other issues. Some of the information the IRS seeks may be used against you later.
Response to Letter 3064C
The IRS may also direct you to do some action. In that case, it's best to get legal advice before reacting. Usually, you'll have to obtain your account transcripts to see exactly what has happened on your case. Plus, it is important to research the reasons given for the IRS action or inaction to determine if you will want to Appeal.
If there's any doubt or you are confused, please follow the Prep Steps for IRS Letter 3064C and see a tax attorney in Colorado Springs, Pueblo or Denver before proceeding.