HCMFAB_B_COMMON Common BAdI for all HCM Fiori 2.0 Apps is the BADI responsible to determine all the process shared by all the apps. For example the manager retrival method.
There's a fall back class released for this BADI, CL_HCMFAB_FB_COMMON Class for BAdI: HCMFAB_B_COMMON. You can change the system behaviour by implementing the BADI.
You can find details in the Backend system in the BADI and the Interface Documentation → TCODE SE18 → BADI Name HCMFAB_B_COMMON → Display → Access the Badi Documentation and navigating to the interface documentation will show the technical details.
2671749 PA-FIO-LIB Change Texts / Translation of HCMFAB APP in FIORI 2.0 Problem Released to Customer