VA's Duty to Assist assists you in getting evidence, such as medical records, to help support your case and only applies during your initial claim and if you file a Supplemental Claim.
Are you going to request  Higher-Level Review or Board Appeal?
Yes,  am going to request a Higher-Level Review.
Your Duty to Assist will not apply; but the reviewer or judge will make sure the VA met its Duty to Assist during your initial claim.
Is there anything I should look out for when I request a call with a senior reviewer?
Yes, you should expect a call at the phone number you gave them or the one your representative gave them on the Higher-Level Review form in which they will schedule a time to discuss your case with you.
What if i disagree with a Board decision?
Your decision comes with the information you need to address errors.
What happens if I miss the deadline for submitting my request?
It is in your best interest to submit your request by the deadline to receive the maximum benefit if your case is granted.
What does the new and relevant evidence help with?