Seal arrangements are based on a variety of criteria; the most common are summarized below.

single mechanical seal: The lubrication film needed between the slide faces is created by the medium to be sealed.

double mechanical seal: The lubrication film is created by the buffer medium. The buffer medium is under a higher pressure than the medium to be sealed.

Combinations: Here are just a few examples from the host of possibilities:

tandem mechanical seal. A frequently employed combination of two single-acting mechanical seals placed in succession. The lubrication film between the sliding faces of the (primary) mechanical seal on the product side is created by the medium to be sealed and, if necessary, is stabilized by a quenching medium. The latter is under a lower pressure than the medium to be sealed.
Three single-acting mechanical seals placed in succession (a "triple tandem") or combination of a double-acting mechanical with a single-acting mechanical seal, e.g. for gradual pressure reduction when working with very high sealing pressures or for safety reasons, e.g. in nuclear power stations.
It is possible to fabricate all mechanical seal combinations as cartridge units; in this form they can be tested for leaks before fitting, and they also preclude installation errors.

Single mechanical seal
Internal arrangement, externally pressurized
Stationary seat
Springs in contact with product

Rotating counter ring
Springs in contact with product

Springs facing away from product

External arrangement, internally pressurized
Stationary seat
Springs facing away from product