Addenda or Addendum

What is the Difference Between Addenda and Addendum?

When to Use Addendum

An addendum is a note at the end of a piece of text, like a letter, a book, or an article. Authors might include an addendum if they notice an error in something that they already sent out or published, or if new information comes out, or if they simply forgot something.

The word addendum is a singular noun

When to Use Addenda

Addenda is the plural form of addendum. If you have more than one addendum, use addenda, not addendums

Summary

Addendum is a noun that refers to a note added to the end of something. Addenda is its plural. The usage cases are entirely separate; they cannot be substituted for each other in any contexts.

To conclude,

Use addendum in the singular form.
Use addenda in the plural form.