To control the characteristics of the graphite sheets (level of flexibility, lightness, sensitivity, etc.); the graphite sheet manufacturers control the ratio between the amounts of carbon used and the amounts of glass fibres used (in general) trying to keep within a certain set cost and also certain set characteristics of the rod of which includes the backbone, tip action, etc.
Graphite sheets rated at 24 ton will have its content of glass fibres increased while its carbon decreased thus making is more flexible but at the expense of weight. 40 ton graphite on the other hand will have a decrease in glass fibres and increase in carbon making it rigid, less flexible, but lighter.