Stock/Assigner/StockUnitAssigner.php
protected function getAssignments($item)
    {
        if ($item instanceof ShipmentItemInterface) {
            $item = $item->getSaleItem();
        } elseif ($item instanceof InvoiceLineInterface) {
            if (!$item = $item->getSaleItem()) {
                return null;
            }
        }

        if (!$this->supportsAssignment($item)) {
            return null;
        }

        return $item->getStockAssignments()->toArray();
    }
protected function getAssignments($item)
    {
        if ($item instanceof ShipmentItemInterface) {
            $item = $item->getSaleItem();
        } elseif ($item instanceof InvoiceLineInterface) {
            if (!$item = $item->getSaleItem()) {
                return null;
            }
        }

        if (!$this->supportsAssignment($item)) {
            return null;
        }

        return $item->getStockAssignments()->toArray();
    }
Returns the item's stock assignments, or null if not supported.

@param mixed $item

@return null|StockAssignmentInterface[]