It is the most common and effective configuration for a large part of the applications for mechanical seals.
It is defined as internal because it is completely immersed inside the process liquid, this allows the sealing surfaces to be cooled and lubricated optimally.
The seal is sized so that the resulting force from the pressure of the liquid to be contained on the thrust surface can ensure contact between the rotating and stationary rings.
The single seal can be used in a rotating configuration, meaning with springs/bellows attached to the rotating shaft, or stationary, where the springs/bellows are installed on the fixed part of the machine.