From Nookipedia, the Animal Crossing wiki
Trustees are users who have their edits automatically patrolled. Users who have displayed experience, contribute large volumes of edits, and whose edits may no longer need to be individually patrolled may be made a Trustee by a Director, a Bureaucrat or an Administrator at their discretion, in order to alleviate the workload of our Patrollers and other staff members. Similarly, Directors and Bureaucrats may revoke trustee status if a trustee's site activity or quality of edits decreases.
Staff members may be placed in this group if they become inactive. If they resume their wiki activity, they may return to their previous position.
A Trustee is not a staff member and does not get any extra tools or editing features. While excellent editors, trustees are not necessarily better editors than non-Trustees; editors are promoted based on both their editing experience and volume of edits on a need-be basis, depending on the current patrolling workload for staff. Therefore, editors cannot apply to be a Trustee. Editors who feel they are ready for a staff position may apply for the Patroller position.