A more thorough explanation:

Definition: QEF is an abbreviation of the Latin phrase "quod erat faciendum," which means "which was to be done."

Example: After completing the experiment, the scientist wrote QEF at the end of his notes to indicate that he had achieved the desired outcome.

Explanation: The example shows how QEF is used to indicate that a task or goal has been accomplished. In this case, the scientist had a specific objective in mind when conducting the experiment, and he wrote QEF to signify that he had successfully achieved it.