Note: .ack is no longer a format that is used. It was discontinued in 2013 with the removal of EOL (Online Plus).
| Extension | Meaning |
| .resptax | The response from the CRA for T1 |
| .tax | The T1 tax return |
| .tfx | The T1013 EFILE format |
| .taxX | The T1 tax return that was sent via a batch, where X is a number that represents how many times this return has been attempted to EFILE. For example, if you see .tax, .tax0 and .tax1, this return has undergone EFILE three times. |
| .resptaxX | Same as .taxX; multiple responses |
| .eax | The TP1 tax return |
| .eaxresp | The TP1 response from Revenu Quebec |
| .eaxX, .eaxrespX | Multiple transmissions and responses |