At Admin Panel -> Misc -> Advanced Options (note: in the 5.0 and earlier series this was on the sponsorship settings page), you can select to make the sponsorship system paid (normal default) or unpaid. Please note that unpaid sponsorship is only available for article type sponsorship, not usergroup sponsorship.

Unpaid sponsorship means that the user can simply click to promote their article to the sponsored type at will. Some people want to use this as a way to encourage people to frequent their website. The sponsorship might be set to last for a day, for example, so that the article owner would have to login every day to keep the article sponsored.

After changing the advanced option, go to Admin Panel -> Settings -> Sponsorship to set up the sponsorship(s).

Of course, the vast majority of people will only be interested in paid sponsorship.