What does the sentence "preventing the use of characters numbered over 0xFFFD" means?

Question

Character encoding

Good afternoon!
I'm reading about MySQL encodings and came across a good article. The author makes the following suggestion:
Starting with MySQL 5.5.3, you should use utf8mb4, not utf8. Both of these groups refer to UTF-8 encoding, but the older utf8 has MySQL-specific restrictions that prevent the use of characters numbered over 0xFFFD.
Can you please explain what the end of this sentence means ( preventing the use of characters numbered over 0xFFFD )?
What does it mean?
Thank you!

Answer 1

Each character in unicode has its own numeric code. 0xFFFD is a number in hexadecimal (65533 in decimal). The old encoding cannot process characters that have a larger code.
www.unicodemap.org/details/0xFFFD/index.html

Answer 2

From Wikipedia :

Basic multilingual plane (BMP)
Character range U+0000…U+​FFFF
The base plane includes many different scripts, including hieroglyphs, and they should be enough in 99.9% of cases.
So if the software uses some scripts that are not included in the base plane, then instead of utf8, you should use utf8mb4.