According to docs:
To hash a new passphrase for storage, set salt to a string consisting of [a prefix plus] a sequence of randomly chosen characters ...
and
In all cases, the random characters should be chosen from the alphabet ./0-9A-Za-z.
Why so?
Also, why the length of that random character sequence is also limited?
One-way function Prefix Random sequence
SHA-2-512 ‘$6$’ 16 characters