Basically, the difference comes down to who controls the status.
When a user is marked as inactive, that's an action taken by a user in your school. Your school has total control over the status.
A user can be marked as inactive at any time. In some cases, it might be the only option to prevent a user from appearing in the application to other users. For example, if a staffer has completed appointments, they can't be deleted. To allow that would cause appointment history to become inaccurate. So setting a user to being inactive might be your best choice to prevent them from using the application anymore.
Soft-deleted on the other hand, indicates the user has been adjusted due to an import from a CSV file or from Clever. Click here to read more about this topic.