The “subject”, in the AAM context, is a general definition for a role, individual user, visitor, or default access to everybody. In other words, the “subject” is who you manage access to.
Each time you change subject on the Users/Roles Manager area, the Current Subject top banner is adjusted accordingly. This gives you the visual confirmation for who are you managing access to.
The current subject can be highlighted with two colors. The red color means that you are managing access to the highest user level (typically Administrator role or users that belong to it) or to the default access for everybody. That is why be careful as you can accidentally restrict access to your user.
The blue color means that the subject has lower user level than the maximum defined. For example, in the default WordPress installation, there are 11 user levels from 0 to 10. In this case, any role or user that does not have user level 10, will be highlighted with blue.
Notice! Access to AAM UI is customizable and you can grand access to certain features for anybody who is registered on your website. For example you can give ability for your editors to manage access to Posts & Pages. For more information about this feature please check How to manage access to AAM page for other users article.