All you need to create a pattern are the Essentials measurements. If you'd like some more design options, including the ability to create hourglass-shaped patterns with waist shaping, you'll need to add the Extras measurements to your measurement set.
The Extras circumferences give CustomFit a good picture of what your body is shaped like, and they allow you to make sweater patterns of four different lengths. Here's a video to learn how to take them.
The Extras lengths are largely based on your personal preferences. Choose sleeve and hem lengths in line with sweaters you like to wear. Here's a video to learn how to take these measurements.
You must enter either all of the Extra measurements or none of them. Once they are entered, they cannot be edited.