Contacts are friendly, allied, neutral, unfriendly, or hostile entities. How, and where, they appear depends on your detection.
Known contacts are either detected completely or set by the scenario designer as detected. They represent locations and facilities that you, the player, know about. Allied units are also known.
This is an unknown air contact, BOGEY. The emissions data is shown below the unit. The yellow bounding shape is the zone of uncertainty. The longer the unit is detected or with better sensors, will help to resolve this.
Selecting the contact will provide information as to how it is being detected and by whom.
Unknown surface naval contacts are known as SKUNKS.
Getting closer or using better sensors will drill down the contact type.
Unknown subsurface contacts are known as GOBLINS.
Unknown ground contacts are known as FIXED or MOBILE.
Detecting these is now even more difficult with the added terrain model.
A contact may need better resolution to be engaged or identified. Either use better sensors, close the range to the contact, or in the case of a naval vessel, slow down. Detecting and identifying GOBLIN contacts is particularly difficult and deserves a tutorial of its own.