Resolver a.k.a. synchro is a position sensor born long time before encoders. They are fully passive components consisting only coils and iron which makes them very robust in harsh conditions – including hot running servo motor under vibration, dust and dirt.
Despite being largely replaced by encoders, resolvers are still surprisingly popular even in new motors. Infineon, one of the largest microcontroller companies, even introduced a new MCU with integrated RDC (resolver to digital converter).
Resolver should be interesting also for DIY people as eBay is full of perfectly fine servomotors equipped with resolvers for bargain prices.
Argon supports resolvers with help of a small adapter board that hooks directly to the feedback device connector. Testing of this adapter however has to wait until final drives are received as pin-out of the Argon feedback port was modified to be more soldering friendly and the adapter in the photo is already designed for the new pin-out.