Deactivated users automatically become community members to protect you and your site. Community members, while they can view content on your site, cannot make any changes to content. Changing a deactivate user's status to community member allows you to keep a user on a probational status, helps you preserve limited pro member seats and protects you against failed log-in attempts.
Use cases for inactive community members
Formerly deactivated pro members are automatically put on a probational status as community members when reactivated. As community member, the reactivated user can view public content but cannot make any changes until changed back to pro member.
Changing pro members to community members upon deactivation allows you to conserve pro member seats when your site has a limited number of seats available. By deactivating a pro member's account and deactivating the user as a community member, the user will not count against any available seats.
For your site's safety, a user account with multiple failed login attempts is set to inactive. When a user is deactivated, group associations and permissions remain valid but stay "dormant" (i.e. the community member has no active permissions, except for viewing pages) until the user is promoted to pro member again.
Change an inactive user to an active user by editing the user's account.