Nearly all collars made are designated as Full Face, half Sweeney, or Full Sweeney
A rule of thumb to determine which is best for your horse, pony, or mule, is as follows:

Full Face: These are designed for the thinner necked horses, such as Standardbreds, Mules, and any animal that has less flesh between the point of the shoulder and the withers

Half Sweeney: The bulk of work horses, as well as many driving horses such as quarter horse and Morgan, will fall into this category. This is for the horse that is fairly average in build

Full Sweeney: Horses with Stud like necks, that is, more bulk between the point of the shoulder and the withers.

As mentioned, this is a "rule of thumb". Your particular horse, pony, or mule, may vary from this guide.

The Sweeney itself refers to the amount of padding or stuffing in the collar for the area just above the point of the shoulder.
Keep in mind that a thin necked horse with more muscling will probably feel better in a half sweeney.