Use the Opening Inventories step to set starting stockpile volumes, tonnes and grades.
When editing stockpile inventories:
Stockpiles populated with data are colored blue.
Press Import from SPCSV your site stockpile current state data, if any.
Press Clear Selected Parcels to reset a parcel to zero.
Press Clear All to reset all stockpiles to zero.
Make sure that all product fields are populated in a stockpile; do not leave zeroes if there should be a value!
Closing inventories can be found in Schedule > Reporting > Inventory Report.