There are two levels of approval possible. Both of them can be activated in the 'Advanced settings':
The first level, that we call the approver, can be set as following:
If the second level of approval - what we call the controller - is activated as well, it will follow the same option you have chosen for the first level of approval.
Rydoo will detect if an approver is able to approve his/her own expenses. In this case you have the option to set up another approver for the expenses of the approver: