First I would like to say that all three transactions used in goods movement.

- MB1A is used for goods issue. This is used for posting materials from stock to consumption say to a cost center(mvt 201) or to a subcontractor(543). Goods issue leads to a reduction in Stock. Transfer posting is used when there is a change in Stock ID or stock category of a material. There is no need for the actual (physical) movement of goods.

- MB1B is used to perform tranfer postings and stock transfers. Stock transfers refer to physical movement where as in transfer posting physical movements of goods is not involved.
eg: transfer posting from material to material (309).
transfer posting from stock to stock (321,343,349).
stock transfer from plant to plant (301).
stock transfer from sloc to sloc (311).

In the above postings if the valuation change is involved then an accounting document is generated.

- MB1C is used for other goods receipts. That is say for a goods receipt w/o reference to PO. Other is used in instances where there is no reference document. Alternatively the transaction MIGO, can be used for all three transaction but when an entry is to be made for the respective movements, specify the transaction and reference in the respective fields on the initial MIGO screen.