This is the number of MIF files (containing SMS hardware inventory records) waiting in Inventory Data Loader's input queue ...

This is the number of MIF files (containing SMS hardware inventory records) waiting in Inventory Data Loader's input queue the last time Inventory Data Loader scanned the queue, minus the MIF files processed since then. When many MIF files are being written to the input queue, this counter is too low until Inventory Data Loader scans the input queue again.