When we navigate to Admin -> Groups and select any group. Within this group, when we go to the Group's Access Rights tab and then select OBS Unit sub-page, we are able to view all the OBS Unit level access rights here that are linked to the Resource Object. However, within the OBS Unit filter field, can only see the Resource OBS and cannot see the OBS which are not linked to resource object.
Even though the OBS is marked as "Use for access rights", under the OBS unit filter in groups access rights, the filter will only show the OBS which are linked to Resource object.