Semi block letter format follows the same arrangement as the block format (i.e., left alignment) except for paragraphs where indention is applied, unlike in block format where paragraphs follow same alignment.
Semi-block formal letter format has been used in the past under name “indented style” of the formal letter. This formal has been utilized mostly in non-official business correspondence, and therefore it was considered as less formal. Today, the email messages have replaced this format.
As shown on the picture below in the semi block letter format each paragraph is indented for a 5 character spaces. The rest of the paragraph is full out.