What is Letter of Authorization?

Hi I’m new uk seller. In identity information, I have just uploaded HMRC welcome letter with my UTR number, after a few hours it returned and there is a button upload “Letter of Authorization”..

I am a Sole Proprietor only, haven’t registered any company, do I need to upload? I saw a template there, what should I write in “name of company”? and “name and function”? All fields are my legal name only?

Thank you!