Manage cost price in product master

In Release product, we have a tab called manage cost and we can enter the price there.

When does Ax use this field if we set an item as moving average ?

For moving average price, does the price varies from purchase price from PO ?

If so, what is the purpose of this field ??

The field will be used in a variety of places, and it will depend if you update the cost on how this reacts as well. In essence AX will record the cost and moving average from the PO but it may not update the cost on the item record. Different costing methods also use it differently so you have to look at how the field is used by the system. So in FIFO it will hold the average cost (if you update it) and use this on inventory transactions, then adjust to FIFO when the inventory close is processed (as one simple example).