There are a few more AP subjects offered than SAT Subjects. Computer Science, Environmental Science, Statistics, European History, and Art are four popular APs that have no SAT Subject Test counterpart.
Where AP and a Subject Test are on the same topic, the content tested on the AP multiple choice questions is similar to the content tested on the SAT Subject Tests. The biggest difference is that in the AP written sections students are asked to invoke large swaths of knowledge and write answers that blend them. A chemistry prompt about molecules could demand an answer about processes. A US History prompt about the Post Civil War era could ask for a comparison to the Post-World-War II era.
Students likely need a deeper understanding of the subject to get a high AP human evaluation than they need to get a high multiple choice SAT Subject Test Score. For this reason, the AP is usually considered “harder” than the SAT Subject Test. However, a student who is knowledgeable AND a good writer under time pressure might earn a 4 or 5 on the AP while having an SAT Subject score that is below 650 because that student’s multiple choice abilities are not strong. Since the SAT Subject Tests have no written sections, the good multiple choice tester may be headed to a better SAT score than on the same AP subject. Indeed, some very good multiple choice testers will “logic” their way to right answers without truly knowing the subject matter. That can only occur on 50% of the AP test though – you are not going to logically “fool” an AP reader that you know a subject in depth.