CP11

• Balance due adjustment notice
• IRS changed your return, resulting in you owing money

CP12

• Adjustment notice resulting in a refund
• IRS corrected calculation errors on your return

CP14

• First balance due notice
• You have unpaid taxes on your account

CP21

• Changes made to your tax return
• Examination adjustments resulting in a balance due

CP49

• Overpayment applied to other tax liabilities
• Your refund was applied to other IRS debts

CP90/CP297

• Final Notice of Intent to Levy
• Notice of your right to a Collection Due Process hearing

CP501/CP502/CP503/CP504

• Series of balance due reminder notices