What is Bai-Istijrar? How to investment made under Bai-Istijrar mode?

BAI-ISTIJRAR:

MEANING AND DEFINITION     

Meaning
 The term "Bai-Istijrar" has been derived from Arabic words بيع and جر (Bai and Zarra). The word بيع means to purchase and to sell and the word جر means to hoist, to lift up, pick up, bring up. "Istijrar" (استجرار) means to purchase goods from time to time in different quantities. In Islamic jurisprudence ‘Istijrar’ is an agreement where a buyer purchases something under a single agreement in different instalments. However, no offer and acceptance or bargain is required each time. The deal will be considered as a single agreement where all terms and conditions are finalized.
 Definition
"Bai-Istijrar" is called such a buying and selling where a person keeps  on taking delivery of required commodities part by part from time to time from a supplier and no offer(Ijaab) & acceptance (Qubul) and bargaining between them  is taken place each time of making and taking delivery.

TYPES OF ISTIZRAR: