Third-party T-series codes
From Sega Retro
When Sega was producing video game hardware, the company used T-series codes to identify officially licensed third party publishers for its systems. T-series codes typically appear on the boxes and/or media of third-party Sega software, in a similar manner to how official G-series or MK-series identifers are displayed.
T-series codes came into use during the late 1980s and were issued until the mid-2000s. Prior to this there were relatively few third-party publishers and no official identification system - Sega typically published all SG-1000 and Sega Master System games (notable exceptions include Salio and various Western companies involved in SC-3000 development), however this changed by the release of the Sega Mega Drive. The first T-series codes appeared publically in 1989.
The first two (or three) digits after the T represent the publisher. Any numbers after this represent the specific release, and a letter on the end specifies a console (the latter not coming into use until the mid-90s).
Not all publishers followed these rules, most notably Electronic Arts which did not show its T-series code on Mega Drive releases.
Mega Drive ROM headers of games developed by a third party (except those that Sega contracts out, such as Nextech) usually contain a field stating the T-series code. For example, the sixteen bytes at offset $110 (hex) of game ROMs would usually look like one of
(C)SEGA year.mon (C)T-xx year.mon (C)T-xxxyear.mon (C)Txxx year.mon
(only found in the ROM header for Fatman, which was published by Sanritsu (T-44))
(later EA Sports Mega Drive titles have T-172 serial numbers in their ROM headers...)