im trying to understand what the f I need to add to my struct that will be hashed and signed

im looking at https://eips.ethereum.org/assets/eip-712/Example.js but I don't understand whats the point of adding Person and Mail in const typedData

I understand the domain key has to be there, and the message can be whatever app specific stuff you want

I just don't understand why we are putting this extra sstuff in the types key