We know that Nintendo-published carts are always (World) because each region matches the others almost ubiquitously, but third-party carts frequently have changed data between regions. Similar to previous console generations, one region might have more languages than another, one region might have a unique publisher, and/or one region might have updates on-cart that the other doesn't. So we cannot assume regions for third-party carts. Here's an example of just how varied each Switch cart region can be:
Therefore, as per No-Intro tradition, assigning multiple regions requires redumps as proof. In other words, a single third-party -USA cart dump should not be assumed as (USA, Europe) without a separate -EUR redump added first. And, it should not be assumed as (World) unless there are redumps from three matching regions. The only time a region should be assumed is when there is insufficient info (like when no cart pictures are uploaded at all), and that should only act as a placeholder until at least one cart region can be determined.
Frequently, carts with a specific region code are sold in other territories, or match other regions. Like -USA carts are often sold in Canada with French on-cart, or -CHT carts are often sold in South Korea with Korean on-cart, or -USA carts might very well end up matching the -EUR carts. But for the sake of consistency with our historical database, that shouldn't influence how we assign regions on DOM. It doesn't matter what languages the cart contains, as we need to apply the universal standard established by Nintendo that is both reliably determined and uniformly applied. So for Switch carts, the assigned region(s) on DOM must be the printed cart region(s) of the submitted dumps, nothing more.
Here is a typical Switch cart:
If this is the only dump so far, the -JPN in the cart serial means it should get assigned the (Japan) region tag. Not (Japan, Asia), not (Japan, Taiwan), not (World), not (Asia), just (Japan).
Here is a glossary of each known region tag and how it should be applied:
If Nintendo-published game = (World)
If non-Nintendo and 1 dump:
-ASI = (Asia)
-AUS = (Australia)
-CHN = (China) (Referring to Tencent's mainland China releases)
-CHT = (Taiwan, Hong Kong) (xuom2 just added this tag today)
-EUR = (Europe)
-JPN = (Japan)
-KOR = (Korea)
-MSE = (Middle East, Southeast Asia) (note: tag doesn't exist yet)
-RUS = (Russia)
-UKV = (United Kingdom)
-USA = (USA)
If non-Nintendo and 2 matching dumps from alternate regions:
---If -USA and -JPN match = (USA, Japan)
---If -USA and -EUR match = (USA, Europe)
Etc.
If non-Nintendo and 3+ matching dumps from alternate regions:
---If -USA and -EUR and -JPN match = (World)
---If -USA and -EUR and -CHT match = (World)
Etc.