What is this about? (or why is it needed?) Due to changes in the way that Google and Microsoft scan their mails looking for possible malware links, we have seen cases where the scan process actually triggers an action, in our case the action behind the button is triggered, so a leave request or a expense approval request may be approved by the email scan process.
What the button does This setting controls whether the system requires a re-confirmation step when approving a request via email.
What the result is
If No - The user will be prompted to approve the request contained in the mail (no change with this). The request will be transferred to the company web site where the user will be prompted to approve the activity at that point.
If Yes - Things will continue as before. Users approving an action in their mail will receive an automatic confirmation be a redirection to the web display.
Do I need it? See What is this about (above). If you find that users complain about approvals "automagically" occurring, ie managers have not approved activities, but they are marked as approved, then you should test this option to see whether the change resolves the issue.
Can I partly implement it for some users? No, it can only be activated for the site.