Hi, INVREVAL-/+ is usually created when there's a revaluation of the inventory as the description says. There can be many situations where a revaluation can be triggered. But looking at your situation related to purchasing I wonder whether this is something to do with a revaluation triggered at supplier invoicing. For example, it could be that you have invoiced goods on different value than it was received from the purchase order. 

Anyways, can you tell us the cost settings for this part in the same site? I believe it is Weighed Average and Supplier Invoice Considered. You can check this in Inventory part > Cost tab.

Thank you vimukthi, 

Cost settings as below

There as no value difference with the PO and the supplier invoice too.   

Had the privilege of investigating this issue at Support.  
It was revealed that there had been a WA calculation which UNRCPT has been calculated from its origin PO (as IFS normally does), but by that time the inventory value has been changed due to another PO. So to balance this off an INVREVAL has been created by the system for the UNRCPT.