What is the 3-letter code on a bolt?

Excuse me, could you clarify something for me? In the context of a bolt, what exactly do you mean by the "3-letter code"? Is this a specific industry term or perhaps a reference to a certain standard or specification? Could you elaborate on the context in which you're asking about this code, as it's not immediately clear to me what you're referring to? It would be greatly appreciated if you could provide more details so I can give you an accurate answer.