What is the meaning of service by post as per provisions of the General Clauses Act, 1897?

"Meaning of Service by post" : According to section 27 of the General Clauses Act, 1897, where any legislation or regulation requires any document to be served by post, then unless a different intention appears, the service shall be deemed to be effected by: (i) properly addressing (ii) pre-paying, and (iii) posting by registered post. A letter containing the document to have been effected at the time at which the letter would be delivered in the ordinary course of post.