Quick answer
When someone requests leave, the approver gets an email with Approve and Decline links, so a decision takes one click without signing in. You can also review everything in the Inbox.
Find it under
Open the Inbox
Click the Inbox icon (or the Inbox tab) in the top navigation. The Waiting tab lists each request with the dates and number of days.

Approve or decline
Use the green tick to approve or the red cross to decline on a request. Approving deducts the days from that person’s allowance and emails them the decision. Declining emails them too, so nothing is left hanging.
Or decide from the email
Every request email contains Approve and Decline links. Clicking one records the decision straight away, which is handy when you are away from your desk.
Who gets the request
It goes to the person’s assigned approver. If their approver is set to All admins, every admin is notified and any one of them can decide. Set a specific approver on the person’s record under People.
On Slack too
If your company has connected Slack, the approver also gets a Slack DM with the same one-click Approve and Decline buttons, so a decision is one click wherever you happen to be. See Use Book Time Off in Slack.