Question
How password strength is determined for Plesk 11.5 and 12.0.18 ?
Theory
Passwords strengths is a sum of the following criterias. Where all characters are verified against rule set, for every match it adds some scores/weights defined for different cases.
Here is rules are applied to validate password in Plesk 11.5 and 12:

If a password is 4 or less symbols in length, it gains 3 scores.

If the length is between 5 and 7, then it gains 6 scores.

If the length is between 8 and 15, then it gains 12 scores.

If the length is 16 or more, then it gains 18 scores.

If password contains at least one lower case letter from '
a
' to 'z
', then it give us 1 score. 
If there is at least one upper case letter from
'A'
to'Z'
, then it brings 5 scores. 
If there is at least one number, then it brings 5 scores.

If there is at least three numbers, then it brings 5 scores.

If there is at least one special character from this list (without quotes): "
!, @, #, $, %, ^, &, *, ?, _, ~
", then it brings 5 scores. 
If there is at least two special characters from the list above, then it brings 5 scores.

If there is both upper and lower case, then it brings 2 scores.

If both letters and numbers, then it brings 2 scores.

If there is combination of letters and numbers and special characters, then it will give us 2 scores.

If the sum of score more than 16, the password is
Very Weak
. 
If the sum of scores between 15 and 24, then it is
Weak password

If the sum of scores between 25 and 34, then it is
Medium password
. 
If the sum of scores between 35 and 44, then it is
Strong password
. 
If the sum of score is more than 45, it is
Very Strong
.
Example
Let's look how it works on the password " P@ssw0rd
"
 Length between 8 and 15 (+12)
 At least one lower case letter (+1)
 At least one lower case letter uppercase (+5)
 At least one number (+5)
 At least one special character (+5)
 Have both upper and lower case (+2)
 Have both letters and numbers (+2)
 Have letters, numbers and special characters (+2)
Overall score is 34, which is less than 35. Verdict is Medium