What is the difference between the semi block style and full block style letter?

A modified block style business letter is justified at the left margin with the exception of the return address (if not using letterhead), the reference line, and your closing, signature, and printed name. These are tabbed about one third to the right of the page.

A full block style business letter has all content justified at the left margin, including the your return address (if not using letterhead), the reference line, and your closing, signature, and printed name.