The IRS link below provides more information about the CP05 letter. This letter is usually sent because the IRS is reviewing your tax return to verify:

• The income you reported on your return
• The income tax withholding amounts you reported on your return
• The claims for tax credits you made on your return
• The withholding claimed on your Social Security benefits
• The household help you claimed
• The Schedule C income you claimed

https://www.irs.gov/individuals/understanding-your-cp05-notice