An EFIN (Electronic Filing Identification Number) is the number the IRS assigns to a firm that has completed the IRS e-file application and passed a suitability check, making it an authorized IRS e-file provider; providers need an EFIN to e-file tax returns.
Full guide: how to get and check an EFIN
EFIN vs PTIN
A PTIN identifies an individual paid preparer and expires every December 31. An EFIN identifies the firm allowed to e-file: one EFIN covers every preparer in the firm, and there is no yearly reapplication while the firm keeps e-filing. The IRS says most preparers need both.
How a firm gets one
Through the e-file application in IRS e-Services: the firm lists its principals and responsible officials, each gives professional credentials or is fingerprinted, and the IRS runs a suitability check. The IRS says approval takes up to 45 days and there is no fee. The EFIN arrives in the acceptance letter, and it can't be transferred, even when the business is sold.