Use FIX when you want to make certain that an activity is done. A FIX request will involve one person who performs an activity. In FIX, only the creator and the performer will see information around the activity, except if the activity is to pick-up or drop-off a person, then he/she also will see information about the activity.
Use SYNC when you want the team to self-synchronize around an activity. In SYNC everyone, except those who have reject the request, will see all information related to the activity.