Q. How does an account become Inactive and how does it become Active again?
A. An Active account is simply an account where the user has logged in at least once in the last six months. Conversely, an Inactive an account is an account where the user has not logged in at any point in the last six months.
It's important to note that whether an account is Active is not connected to whether the account is Enabled, Disabled, or Deactivated.
For example, an Inactive account that is Enabled can still be accessed at any time (and may take up a license, depending on a number of factors). But a Disabled account can't be accessed and can't take up a license, regardless of whether it is Active or Inactive.
Put differently, whether an account is Active depends on the user's activity and is outside of the domain administrator's control, whereas other account statuses, like Disabled, are controlled by the domain administrator and do not reflect the user's activity.