Accessory lifts in a strength training program are considered supplemental and are said to serve or assist in the progress of major exercises.
Mark Berry was a National weightlifing champion and was the U.S. Olympic Weightlifing coach at the 1932 Olympics. Berry was also an author and helped shape how people would rethink the use of a barbell in training. Prior to the 1930's the squat, or deep knee bend was considered just another accessory exercise and was used very little. Berry believed in heavy squats for building bulk and power and placing this exercise as a regular part of ones routine.
Today the squat is commonplace and is an integral part of exercise programs. Mark Berry was also a strong advocate of neck training and like the squat it was a significant part of an exercise regime. He felt the single most important method for success was strengthening the neck and spinal cord. The following is a quote from Berry's work.
Do not treat neck training as an accessory lift. If you want to Get Strong strength train the muscles of the cervical spine.
The Pendulum 5 Way Neck