It is a notice that tells a payer that he or she may be responsible for backup withholding. It is accompanied by a listing of missing, incorrect, and/or not currently issued payee TINs. Large volume filers will receive a CD or DVD data file CP2100, mid-size filers receive a paper CP2100, and small filers receive a paper CP2100A.
Large Filer …………. 250 or more error documents;
Mid-Size Filer ……… Between 50 and 249 documents.
Small Filer …………… Less than 50 error documents.
For additional information see Publication 1281, Backup Withholding.