As described here:

https://community.atlassian.com/t5/Sourcetree-questions/Unable-to-quot-Reset-quot-multiple-files/qaq-p/668395?utm_source=atlcomm&utm_medium=email&utm_campaign=accept_as_solution_reminder&utm_content=topic

 

Option "Reset all" is not available when both new and modified files are selected. It would be so useful to have it when we want to "discard all changes". Currently we have to manually select only modified files on the list, which is tedious when the list is long.