Out-of-office approval sharing is typically set up by regular users themselves, whereas normal approval sharing can only be set up by administrators with the "CDC-SUPER" permission set.
Only employees can set up out-of-office approval sharing.
With out-of-office approval sharing, approvals are only shared with the employee's manager, whereas with normal approval sharing, they're shared with a specific user chosen by the employee.
Managers can set up out-of-office approval sharing on behalf of their staff, but not normal approval sharing.