"Headhunters" are full-time dedicated recruitment agents hired by companies to find suitable candidates for job vacancies. They can be self-employed or work for a service provider specialised in the provision of personnel.
"In-house recruiters" are employees in companies who are solely concerned with finding new employees for their company.
There are other headhunters and HR managers active on XING who don’t use the onlyfy TalentManager. These people are not included in the settings you make under "Top recruiters". Your settings for other XING members apply for them.