utf8mb4_general_ci

utf8 - 8-bit unicode transformation format.
mb4 - store at 4 byte characters.
general (or unicode) - refers to the specific sorting and comparison rules.
ci - case insensitive.


utf8mb4_unicode_ci is based on the official Unicode rules for universal sorting and comparison, which sorts accurately in a wide range of languages.

utf8mb4_general_ci is a simplified set of sorting rules which aims to do as well as it can while taking many short-cuts designed to improve speed. It does not follow the Unicode rules and will result in undesirable sorting or comparison in some situations, such as when using particular languages or characters.


There is quite a bit more reading over here:
https://stackoverflow.com/questions...ral-ci-and-utf8-unicode-ci/766996#766996

There are even more recent updates within that link. But be aware that UBB.threads covers an extremely broad range of server setups with wide php and mysql versions. Some are a decade old -- are you still supporting php 5.4/UBBT7.5.4 sites? And some are running the bleeding edge php and mysql versions. The the choice needs to be made on which collations will best cover everyone.

The UBB.threads software has seen a tremendous amount of updates over the past 3 years. Prior to this, it remained near stagnant from 2009 - 2017.