What is the auto-update campaign mode?

Auto-update campaign mode is set with the checkbox in the 3rd stage of the campaign manager.

Auto-update checkbox - if you mark this checkbox, your campaign will be auto-extended whenever you'll try to distribute new code, and the initial batch of codes has already run out. 

For example, if my 1000 vouchers will be already assigned to customers (published, distributed) and I'd like to send it to 1001 customer, thanks to auto-update mode I can do it without any campaign updates. Voucherify will automatically add a new code when I click to send the code from this campaign.