Input subsystem design keyboard driver

Enter the system design keyboard driver-General Linux technology-Linux programming and kernel information. The following is a detailed description. After reading the design of the linux Input subsystem, I have a question: when setting the evbit field in struct input_dev to define the Input type to be accepted, why do I only operate on evbit [0?
The explanation on the internet is that the current event type does not operate on 32, so you will see that operations on the evbit array are performed on the bits in evbit [0], and you do not quite understand it, can someone explain this? thank you!