Take a look at this web page for a very brief description of resolvers and synchros. You will see that, although they use the same concept, there is a difference between them.
Resolvers used to be the mainstay of brushless AC motor drives back when the drives were analog. Resolvers are absolute position devices, a good characteristic for motor commutation. The decoded resolver information also lent itself very well to commutation. And, best of all, a resolvers is just about bullet-proof. It has no on-board electronic components and no fragile mechanical components like encoders do.
However, resolvers do have bandwidth issues that may cause issues in some applications.
With the advent of digital drives the resolver isn't as popular anymore. Digital drives can easily handle commutation based on the variable position offsets that an incremental encoder system may create. And encoders don't suffer from the bandwidth limitations of the resolver quite as severely. So many of the drives today just go the encoder route.
A good rule of thumb is if you need absolute position within a shaft revolution and you need the sensor to be tough, consider resolvers.
Keith