In each admin report plugin, there will be an optional file lib.php, which may define two variables:
$placeintree = array('users', 'roles');
$requirecapability = 'moodle/role:manage';
If this was in admin/report/capability/lib.php, then it would say that the capability report appears under Users -> Permissions, and is only available to people with the moodle/role:manage capability.
If lib.php does not exist, or either of these variables are not defined in that file, then the default is
$placeintree = array('reports');
$requirecapability = 'moodle/site:viewreports';
The strings in the $placeintree array are the names passed to the admin_category constructor.
Note that this has to work, even if reports want to add themselves to the experimental category, that is, reports need to be processed last when building the tree, although the reports category will remain second from last.
As well as moving the new Capabilities report to Users -> Permissions, we will move Unit tests and Functional DB tests (and XMLDB editor, although that is not a report) to a new Developer top level category.