What is Bill of Material (BOM) and Material requisition note (MRN) ?

Suppose you went to a restaurant for giving party to friends on passing CA- Exam.You asked all your friends about the items they want to eat.
You wrote all the items on a paper and gave that paper to waiter to bring food. This paper is called BOM. At end you asked again to all your friends for deserts. You again wrote 1 desert for each of your friend. This paper is called MRN. In costing language, BOM specifies all the materials required for a job while MRN specify single item or few items required to complete a job.