A semi-block letter is less formal than a block letter.
Because the paragraghs are not indented
semi block
It depends on the letter.
Block style Semi block style indented style full block style hanging indented style simplified letter style
Check out the related link for an example of the full block format of an invitation letter.
Yes, commas should always be used where appropriate.The only distinction of a block format letter is that there are no indents at the beginning of a paragraph. All content of the letter is standard.
Block style Semi block style indented style full block style hanging indented style simplified letter style
block style, semi block style, indented style, full block style, hanging indented style, simplified letter style
block and modified block (NovaNet)
See the link below for a sample of the modified block style business letter.
I say that the full block format for a business letter is the most convenient to format. All of the information in the letter is justified to the left margin.