The CAF (Centralized Authorization File) number is a 9-digit number according to the IRS instructions.
Per the IRS Instructions for Form 2848 Power of Attorney and Declaration of Representative, page 3:
Line 2. Representative(s)
Enter your representative's full name. Only individuals who are eligible to practice before the IRS may be named as representatives. Use the identical full name on all submissions and correspondence. If you want to name more than three representatives, indicate so on this line and attach an additional Form(s) 2848.
Enter the nine-digit CAF number for each representative. If a CAF number has not been assigned, enter “None,” and the IRS will issue one directly to your representative. The CAF number is a unique nine-digit identification number (not the SSN, EIN, PTIN, or enrollment card number) that the IRS assigns to representatives. The CAF number is not an indication of authority to practice. The representative should use the assigned CAF number on all future powers of attorney. CAF numbers will not be assigned for employee plans and exempt organizations application requests.
Enter the PTIN, if applicable, for each representative. If a PTIN has not been assigned, but one has been applied for, then write "applied for" on the line.
Check the appropriate box to indicate if either the address, telephone number, or fax number is new since a CAF number was assigned.
Check the box on the line for up to two representatives to indicate that you want original and other written correspondence to be sent to you and a copy to the indicated representative(s). You must check the box next to a representative's name and address if you want to authorize this representative to receive copies of all notices and communications sent to you by the IRS. If you do not want any notices sent to your representative(s) then do not check the box. By checking this box you are not changing your last known address with the IRS. To change your last known address, use Form 8822 for your home address and use Form 8822-B to change your business address. Both forms are available at IRS.gov. Also, by checking this box, you are replacing any prior designation of a different representative to receive copies of written correspondence related to the matters designated on line 3.
Note. Representatives will not receive forms, publications, and other related materials with the notices.
If the representative is a former employee of the federal government, he or she must be aware of the postemployment restrictions contained in 18 U.S.C. 207 and in Circular 230, section 10.25. Criminal penalties are provided for violation of the statutory restrictions, and the Office of Professional Responsibility is authorized to take disciplinary action against the practitioner.
Students in LITCs and the STCP. The lead attorney or CPA must be listed as a representative. List the lead attorney or CPA first on line 2, then the student on the next line. Also see Declaration of Representative later, to complete Part II.
Note that any link in the information above is updated each year automatically and will take you to the most recent IRS version of the document at the time it is accessed.