It will include things like character backgrounds, faults & favors, endurance use in melee combat and chases and a lot of other optional rules. In addition wilderness/hexplore rules, more terrors and other stuff. Why is it in a separate book and not in the core rules? Because they are not strictly necessary to play the game. You may want to add them into your game which is great, but I did not want to overload the core rules nor create so many options one was left not knowing what to do. Putting them into a separate book clearly defines and I think clarified the idea that they are additions, not mandatory.
Also responding to a quick question on Breath Weapons. Below is an assortment of options:
Stream breath weapon
Their range (distance) is 1/4 creatures current ST in spaces, rounded down. This assumes a 5 foot wide, straight line shot, breath weapon.
A ST20 has a 5 spaces (25 feet breath weapon range)
A ST40 has a 10 space (50 feet)
A ST60 would be 15 spaces (75 feet)
A ST80 would be 20 spaces (100 feet)
Cone Breath Weapon
The range (distance) is 1/4 creatures current ST in spaces, rounded down. A cone shaped breath weapon sacrifices 10 feet in distance for every 10 feet in width. Maximum width is 30 feet. breath weapon must travel 10 feet before growing 10 feet in width as shown below
Cloud shaped Breath Weapon
A cloud shaped gas breath weapon is 1/2 the creatures current ST in L x W. It is a 10' cloud in height.
a ST20 cloud breather would be 10 feet long but 10 feet wide
a ST40 cloud breath could be 20 feet in distance, but 20 feet wide
a ST60 cloud breath is 30 feet long but 30 feet wide
a ST80 cloud breath is 40 feet long and 40 feet wide
Hope that helps!