Answer:
When you are not satisfied with any part of it.
Step-by-step explanation:
When writing a letter or any piece of writing, there are several steps to create a good written piece. One which, revising and rewriting is the one.
In the given context, two elements of writing are focussed, that is revising and rewriting. Revising means 'to look again' which means that after finish writing a piece it is important to look at the text again for any grammatical, spelling errors, etc. While revising, many times some errors come and that is where rewriting or editing comes in.
While editing or rewriting a text or letter or any writing piece we fix the problems in spellings, punctuation, capitalization, letter-form, paragraphing, etc. Rewriting or editing is done if there are certain things that you want to omit or add.
So, the correct answer is when you are not satisfied with any part of a letter, or any other writing piece, you rewrite or edit it.