38 CFR § 17.132

When any claim for payment or reimbursement of expenses of medical care or services rendered in non-Department of Veterans Affairs facilities or from non-Department of Veterans Affairs resources has been disallowed, the claimant shall be notified of the reasons for the disallowance and of the right to initiate an appeal to the Board of Veterans Appeals by filing a Notice of Disagreement, and shall be furnished such other notices or statements as are required by part 19 of this chapter, governing appeals.