Visibility check (/admin/plugins/extranet/config/visibilitycheck.action)
Within the visibility check you can test if "User A" can see "User B" in a global context (search, people directory)
Who sees who?
The first picture reads as follows: user "admin" can see user "ssonnenschein". 
This does not mean "ssonnenschein" can also see admin!
The second picture says: user "smeier" can NOT see "pwinter".