Use when an action emphasizes the completion or success of something important. Her brilliant performance set the seal on her reputation as a great actress.
Pro Tip 2/3
Not Literal
It's a metaphorical phrase and doesn't involve an actual seal or stamping action. Winning that award set the seal on his hard work.
Pro Tip 3/3
Emphasizing Result
It highlights that a certain outcome is now definite or assured. Signing the contract set the seal on the company's merger.