Quick Fields uses a queue for processing. When a scan starts, the files are captured from the scan source onto the Quick Fields workstation and enter a queue for processing. If processing stops while files are still in the queue, they will remain in the queue and be available for processing next time you scan with that session. 

You can control how many files are added to the queue at a time using Retrieval Limits. (Here I am assuming that you are using Universal Capture as your scan source). 

If you're concerned about unprocessed files having to be rescanned, you might also want to look at Post-Processing options. You can move the original files to a "Processed" folder after they are retrieved and then delete them later instead of deleting them immediately when they are retrieved by Quick Fields.