Likely Qty Committed.  These are set in Order Entry Details and can be viewed in Location Details by clicking on the header for the Quantity Committed column.

Anything else - the Qty Committed appears to be zero for the items I just checked with status 'On hold'.

It's on the details of the Physical Inventory Quantities screen.  The system will set lines to On Hold if there is no cost.  You can change the status to "Ready to Post" if you'd like the Qty to be adjusted without cost.