This is the same process you can access in the People view with the Scan Collection for Faces button. Here you just cannot set the Fast - Accurate parameter but just the Face Data Management method and the option to Clear and rebuild all training data. For more information about the Faces Data Management options see the People View section.
While the faces management process is under progress, notification on the bottom right of main windows will be visible to indicate the amount of items already done.
To run properly, the process needs the deep-learning models to download at the first run of digiKam. See the Quick Start section for details.