An "advance" is just that: payment advanced against projected sales, and hence, per-volume royalties.
The publisher generally calculates the advance based on projected sales, and that's often all the money an author will see, though if an advance is "earned out" (sales exceed the projection), the author can earn additional royalty payments based on a percentage of the per-volume sales price. The usually cited figure is 10%, though this may vary.
TL;DR: Snowden is guaranteed the advance, but may earn more.
The publisher generally calculates the advance based on projected sales, and that's often all the money an author will see, though if an advance is "earned out" (sales exceed the projection), the author can earn additional royalty payments based on a percentage of the per-volume sales price. The usually cited figure is 10%, though this may vary.
TL;DR: Snowden is guaranteed the advance, but may earn more.
https://www.thebalancecareers.com/book-advances-and-royaltie...