create-or-fill-user is LH_Profile_Create_Or_Fill_User_Ability (includes/lh-profile-create-or-fill-user-ability-class.php). It replaced lh-buddypress-add-user/create-user and needs LH User Provisioning, which resolves existing users by email or registered alias (lh_user_provisioning_resolve_user()) and, from 1.3.3, seeds a new user’s BuddyPress last activity from their registration date without ever bumping an existing value. Its profile field schema comes from LH_Profile_Fill_Fields.search-users is LH_Profile_Search_Users_Ability (includes/lh-profile-search-users-ability-class.php). Since 3.8.0 it is the network’s only user-lookup ability. Other plugins extend it with the lh_profile_search_users_input_properties, _result_properties, _restrictions and _results filters: LH Membership adds membership status, and LH Vcard adds a signed short-lived vcard_url.