SAP’s delivered workflow allows single level approval which works out of box without much configurations. However some clients will have requirements to have more than 1 level of approvers. This customization can be handled using BADI: PT_ABS_REQ, You can activate this BADI using the SPRO or PTARQ tcode under the following node.
Further using the method GET_MULTIPLE_APPROVER we can add any number of approvers based on customized logic. Also based on leave type we can influence of approvers determination logic. Exporting parameter EV_APPROVER_LEVEL should be filled with level of approvers.
Based on PERNR we can use a multitude of delivered function modules to find higher level managers to fill the approvers. This is further explained in the SDN link below.