Page 4 of the rules has an interesting passage that I am having trouble understanding.

Defense tokens begin the game on their readied side.
When a readied defense token is spent, it is flipped to
its exhausted side. When an exhausted defense token is
spent, it is discarded.

When the token is spent, its exhausted, it can't be used gain in the round anyway. How is an exhausted defense token spent? How is a discarded token different than an exhausted token? Is this a misprint? If so, what was it supposed to mean?

You can spend an exhausted defense token later in the same round, just not on the same attack.

Spent exhausted defense token

While neither book has complete disclosure, based on the rules given in both books, you can spend exhausted tokens. Only exhausted cards can't be used again until they're readied. An exhausted token is spent in the same manner as a readied token, giving the ship/squadron the same effects as if the token were readied, except that the token is discarded instead of "further exhausted".

Spending an exhausted token is a last ditch effort, because when an exhausted token is spent, it is gone for the rest of the game. On the other hand, exhausted (red) tokens are flipped back to green in the status phase. So forcing your opponent to spend exhausted tokens can really make a difference in the game. Ultimately, forcing bad choices on your opponent to help you win the game. On the other side, there may be some merit to spending an exhausted defence token if it means winning the game in round 6.

I get it now. Thanks. I see a lot of round 5 and round 6 token discards then.

I can't think of worse possible terms for FFG to use to describe these two different states. Exhausted and Spent are synonymous in dictionaries.

Don't misunderstand what synonyms are. Synonyms MAY have similar or identical meanings, but do not necessarily HAVE the same meanings in all contexts. We often say that we "spend" money, but it's exceedingly rare to say that we "exhaust" money, so those synonyms are not necessarily interchangeable. Game rules, especially in the case of complex games such as this, commonly define terms in particular ways and then use them consistently and distinctly, not synonymously.