Just a guess here, but:
Exact = The most precise solution which requires extra computing power and time.
Raster = The opposite of vectors. Raster images are like photographs, made up of lots of pixels. They are fine at their generated size, but as you resize them, you'll loose information. They likely draw and render faster than the Exact option above.
CGR = There are details at this link: http://www.staff.city.ac.uk/~ra600/ME2105/Catia%20course/CATIA%20Tutorials/draug_C2/draugbt0423.htm