I recently asked [this](https://space.stackexchange.com/questions/59283/what-makes-aluminum-aerospace-grade) question. There, I was also linked to [this](https://space.stackexchange.com/questions/4531/what-exactly-makes-a-bolt-aerospace-grade) other question. Both, however, could benefit from the answer to one fundamental point. What is "aerospace grade"? If sometimes aerospace grade and non-aerospace grade parts are the *same exact thing* then what makes one aerospace grade and the other not? Why would a company pay millions more dollars for one, but not the other?