Differences training is not endorsed in the pilot's license

Differences training is required by FCL.710(b) Part FCL, Regulation Ec 1178/2011. 

Type rating license endorsement considers the license entry with single type rating that covers different variants within the same - common type. Differences training is not endorsed in the pilot's license. If you have not flown on the particular variant for more than 24 months, you have to attend the additional differences training before resuming flying on the particular variant.

Boeing 737 as an example

As a matter of example we can use Boeing 737.

Common license type rating endorsement for Boeing 737 is B 737 300 - 900. The type rating can be however obtained on any of the variants of Boeing 737 - that means on the variants of Boeing 737-300Boeing 737-400Boeing 737-500 which are constituting into the so-called Classic generation.

The Boeing 737 can however be obtained also in simulators constituting into the so-called Next Generation of the B737. That means Boeing 737-600, 737-700, 737-800, 737-900. You can also obtain the typerating in the simulator which represents the Boeing 737 MAX. It is obvious, that those three generations are way different when it comes to the cockpit layout. For that purpose when you are transitioning from Boeing 737-400 to Boeing 737-800, you have to attend the differences training. Nevertheless when you are transitioning from Boeing 737-300 to Boeing 737-400 (varians within the same generation), only so-called familiarization training is required.