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.
If you want to authorize your representative to receive copies of all notices and communications sent to you by the IRS, you must check the box that is provided under the representative's name and address. No more than two representatives may receive copies of notices and communications sent to you by the IRS. Do not check the box if you do not want copies of notices and communications sent to your representative(s).
Note. Representatives will not receive forms, publications, and other related materials with the correspondence.
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. The IRS will not assign CAF numbers for employee plan status determinations or exempt organization application requests.
Enter the PTIN, if applicable, for each representative. If the IRS has not assigned a PTIN, but the representative has applied for one, write "applied for" on the line. A PTIN is required for representatives that are registered tax return preparers and unenrolled return preparers.
Check the appropriate box to indicate if the representative's address, telephone number, or fax number is new since the IRS issued the CAF number.
The postemployment restrictions contained in 18 U.S.C. 207 and in Circular 230, section 10.25, apply to a representative who is a former employee of the federal government. The Taxpayer Inspector General for Tax Administration may enforce criminal penalties for violations of the restrictions, and the Office of Professional Responsibility may take disciplinary action against the practitioner.
Students in LITCs and the STCP. You must list the lead attorney or CPA as a representative. List the lead attorney or CPA first on line 2, then the student on the next line. Also see Part II. Declaration of Representative, later.
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.