The standard SAE tire coordinate system

I have one question that i have found before learning to design solid tyre is :
What is main function of tyre ?.
After i had worked to design and analyze . I found best answer that to descripe the main funtion of tyre form the Fundamentals of Vehicle Dynamics :
Tyre wheel system provides the following three basic function :

1. Support and transfer vertical loads , absorb and reduce ground impact and the consequent vehicle vibration
2. provide longitudinal forces for acceleration and braking
3. provide lateral forces for cornering and steering

Before to design solid tyre . I will be shown the standard SAE tire coordinate system in picture.