All contacts which cannot be technically identified because of user's browser errors are referred to as 'Unknown'.
A percentage of visits lower or equal to 2% is to be considered within the norm as in line with the average of users.
A high percentage of 'unknown' visits (higher than 2%) can be due to the fact that the web site is accessed from different domains and the field 'Domains to be considered' in the account setting has not been filled correctly (or not filled at all).
Filling this field is very important for correct data tracking. It is recommended to read 'Domains to be considered' in the guide.