You would set a to prevent users from immediately changing their password several times in one day to return to the current password. this is particularly important if your password policy has a history depth of five.

Minimum password age

Explanation:

The Minimum password age policy setting determines the period of time (in days) that a password can be used before the system requires the user to change it. You can set passwords to expire after a number of days between 1 and 999, or you can specify that passwords never expire by setting the number of days to 0.

is this supposed to be a question ?

answer; always; never;