use_contacts:

    will use the model calendar.contacts to display a list of friend, coworkers, favorites, ...

    In module calendar, it uses this option for example. You don't want events from everybody by default, but only your colleague from your service

attendee:

   you can give a fields. Usually (user_ids, or partner_ids, ...). For each occurence in this fields, an icon will be displayed at top left of the event. 

avatar_model:

   if you use attendee, you can replace default font awesome icon with a picture / photo. to do it, you can give the model which will be used. Image will be generated with url: /web/image/[avatar_model]/[attendee_id from attendee]/image_small