A more thorough explanation:

A deficiency letter is a communication from a government agency, such as the Internal Revenue Service (IRS) or the Securities and Exchange Commission (SEC), to a taxpayer or registrant, respectively, indicating that there are issues with their tax return or registration statement.

For example, an IRS deficiency letter may inform a taxpayer that their tax return is incomplete or contains errors that need to be corrected. Similarly, an SEC deficiency letter may highlight areas where a registration statement does not meet federal disclosure requirements.

These letters are also known as letters of comment or letters of comments.

Overall, a deficiency letter serves as a notice to the recipient that they need to take action to address the issues identified by the government agency.