Understanding commonly used terms and their acronyms can make it easier to prepare and file your tax return.

Taxpayer Identification Number (TIN): Refers to the number issued by the IRS or Social Security Administration to identify taxpayers. For individual filers, TIN is usually the taxpayer’s Social Security Number (SSN) or individual taxpayer identification number (ITIN). Other TINs include identification (ID) numbers listed below and the Adoption Taxpayer Identification Number (ATIN).

Electronic Filing Identification Number (EFIN): A six-digit ID number assigned by the IRS for e- filing.

Electronic Return Originator (ERO): An authorized IRS e-file provider that originates the electronic submission of returns to the IRS.

Electronic Transmitter Identification Number (ETIN): A five-digit ID number assigned by the IRS to a participant in IRS e-file that performs activity of transmission and/or software development. (TaxAct has an ETIN.) If a customer doesn’t transmit his or her own returns directly to the IRS, he or she will not use an ETIN.

Employer Identification Number (EIN): A nine-digit number used to identify the tax accounts of employers and certain others who don’t have employees.

Individual Taxpayer Identification Number (ITIN): A nine-digit tax processing number for certain nonresident and resident aliens, their spouses, and dependents. The ITIN is available from the IRS only for people who cannot obtain a SSN. (To obtain an ITIN, complete the IRS Form W-7 Application for IRS Individual Identification Number.

Preparer Tax Identification Number (PTIN): An eight-digit ID number that starts with a "P." Anyone who prepares or assists with preparing federal tax returns for compensation must have a valid PTIN, and all enrolled agents must also have a PTIN. The PTIN cannot be used in place of the EIN of the tax preparation firm.

Site Identification Number (SIDN): An eight-digit ID number that starts with an "S" and identifies a Volunteer Income Tax Assistance (VITA) or Tax Counseling for the Elderly (TCE) site. The SIDN is used on every return prepared by any of the volunteers at a particular VITA site in place of a PTIN, which is for use only by paid preparers. (A paid preparer won’t have a SIDN.)

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.