Understanding Serials

(sources: gbatek, 3dbrew)

Nintendo - Game Boy Advance

The ROM chip is sometimes covered up.
Four-letter serial: The first four characters of the large text, and also the fifth, sixth, seventh and eighth characters of the third line of small text, on the ROM chip. Usually (always?) this will be duplicated exactly on the physical media (label) serial.
Revision: The last character of the large text, and also the tenth character of the third line of small text, on the ROM chip.

Nintendo - Nintendo DS

Four-letter Serial: First four characters of code printed on back of card. Usually (almost always) this will be duplicated exactly as the second part of the label serial.
Revision: Sixth character of code on back. This number is in hex, but it doesn't usually matter as it is very rare for a game to have more than 9 revisions.

Nintendo - Nintendo 3DS

Four-letter Serial: First four characters of code printed on back of card. Usually (always?) this will be duplicated exactly on the physical media (label) serial.
Revision: Seventh character of code on back. This number is in hex, but it doesn't usually matter as it is very rare for a game to have more than 9 revisions.

Nintendo - Nintendo Switch

Four-letter Serial: first five characters of the code on the back of the card. Usually (always?) this will be duplicated exactly on the physical media (label) serial e.g. LA-H-XXXXX-USA.
Revision: Last three characters of the back code.