All the internal workings of the system should be ignored as the user wrote above - On a high level, functional spec should contain - 1.) Use cases (including the cases where there is error) 2.) Data description (if needed) - length of input, relation between data etc. 3.) Critical business rules or algorithms (optional – depending on the product) 4.) UX design (can be a separate document) The document can also cover performance, security concerns depending on the product. A Functional spec should be as detailed as per the need of stakeholders, developer and testers. At the end of the day - PM is responsible for any misses.

It really depends on the type of product you're developing..and each company has a different approach on what's important in the doc. So unless you know what is important to Amazon, answering this question can be tough.