Use the below mentioned SAP link to understand it in detail . It is clearly mentioned in the help that "if there is any change in the stock data of a material and that is affecting bookkeeping information then system generates PCN" e.g. when you do a SLoc1 to SLoc2 transfer at IM level system transfer the stock from SLoc1 to SLoc2 but the WM is still pending and to perform the WM activity you need to create a TO against the PCN and complete the activity
Posting Changes - Warehouse Management System (WMS) - SAP Library