On a typical flexible pavement section, what is the difference between subgrade and subbase?

1. They are the same layer; the names are interchangeable
2. Subgrade is the prepared native soil or compacted fill that supports the pavement structure; subbase is a specified imported layer placed on the subgrade
3. Subbase is always the existing asphalt surface; subgrade is the tack coat
4. Subgrade is placed above the asphalt surface course