TCM0014160 - Tax Credits Manual - HMRC internal manual
If you are dealing with a new settlement case, go to Step 2. If you are dealing with a returned form TC621, go to Step 6. If the BF date for form TC621 has expired, go to Step 16. If you have received a letter from the customer withdrawing from a settlement agreement, go to Step 16. If the BF date for a form TC622 has expired, go to Step 17. Step 2 Check the form WTC/AP or the Appeal letter and identify the reason why the customer is appealing check if the decision they are appealing against is correct. To do this obtain all relevant evidence by checking some or all of the following
Source: gov.uk