What's `sortlen` of mysql collation? Collation                | Charset  | Id  | Default | Compiled | Sortlen |
+--------------------------+----------+-----+---------+----------+---------+
| utf8_general_ci          | utf8     |  33 | Yes     | Yes      |       1 |
| utf8_bin                 | utf8     |  83 |         | Yes      |       1 |
| utf8_unicode_ci          | utf8     | 192 |         | Yes      |       8 |

What's sortlen of mysql collation?
What's the difference between sortlen of utf8_unicode_ci and utf8_general_ci?
The field Sortlen indicates the bytes needed when collating data:

So the collation is used to sort and compare the strings presented, but as the complexity of the rules for a language increases, you need space to sort the string or compare, because you have to store some information on the way.
Rick James is right you can see them,. because they are used on inside, when sorting or comparing and the number of bytes for sorting are set when the rules are made for a specific.