It's not currently possible to select multiple files or folders and classify them all at once. We have to click the More Options button on each item to apply the labels one-by-one. I need to apply classifications to about 40-50 existing folders so I'll have to do them individually. Please consider allowing us to click the checkboxes next to multiple items and then show the classify option under More Options, or add a button alongside there next to the Move/Copy, Collections, Trash buttons, etc.

Update: I discovered that you can use a Box Relay workflow using a Manual Start trigger to apply a specific classification label to multiple files, but this does not work for selecting multiple folders.