A site is the advertising medium or advertising vehicle. For example, a website or magazine is considered a site.

Overview of a Site Record in ADvendio.com

Each site record consists of the site detail section where you find buttons for basic functions, such as editing the site details.

Below this section, the related lists are displayed. They contain items assigned to the site. To add further items, use the New button.

Mandator & Contact Person

Mandator is the account owning a site. The Contact Person is an employee of the Mandator. 
If you assign a Mandator and a Contact Person to your sites, you are able to run reports analyzing all site-based deliveries of one mandator.