when i going to create a Firm planned production order first i filled Header , Then i go to refresh function , then automatically Prodcution Line will update .
Here i observed one thing That is Unit cost field when refresh then Unit cost field is getting the Value from the From Unit cost in Item card .
But i run the funtion Update Unit cost from Firm planned production order then the Unit cost is changing in Firm planned production order .
If you want an answer to this sort of question you really need to speak to a developer to analyse the code and how the refresh calls this.
Otherwise you need to give more information regarding versions and circumstances.
I just picked any FIFO item I had, loaded it onto a production order, hit refresh, then replan and the cost never altered, it was still correct.
Why are you worried about it anyway? You are in FIFO so the cost roll up on production completion will update the cost from average and be pushed through to the accounts when finished, the field actually means very little, it is just an indication of the current cost of the item, not the cost it will actually be.
My answer is still the same, ask a developer to go through what the function is doing.
I would guess it will go down through all of the levels, pick the average cost of every sub-assembly and item and then bring it back as the unit cost, this will differ in a FIFO environment because of timing issues between raw materials, sub-assemblies and finished goods.
Look at the average cost of every item used in production - total them, it will not match the current average cost of the finished product I would guess.