The term "XXL Model" refers to any model with a base too large to fit in one Square on the Mission Map. Their bases are normally twice the diameter of standard Models, and thus take up two by two Squares on the Mission Map.
There are no Hero Models of XXL size, only Alien Models, and in the Core Box, only the Overmind is of this type. There are various other Alien XXL Models available in Expansions and stretch goals.
In order for a Hero to have Line of Sight to an XXL Model, it is enough that there is Line of Sight to one of the Squares the XXL Model occupies.
XXL Models are immune to Special Combat Effects; they can't receive the Knock Down, Shaken, Stun or Push Back Special Combat Effects.
XXL Models are too large to fit through most normal building doors on the map, but XXL Models can use a Destroy Wall action to create an opening two Squares wide. (Use a Ruined Wall Token to mark this.)
There are a few different XXL models available for the Alien Forces:
There is one XXL model available to assist the Hero Players:
Model
The term "Model" in Chronicle X refers to an individual character (human or alien) and the miniature used to represent it on the Mission Map.
For the Alien Forces, Alien Models can be activated together as an Alien Unit, or they can move individually.
For the Hero Players, the Heroes can only be activated individually.