Allowing objects to be archived
To permit an object to be archived (also known as serialisation) its class needs to implement the NSCoding protocol. This protocol exposes two methods: -encoderWithCoder and -initWithCoder:. By implementing these methods, your object can be serialised/deserialised (for plists or RPC) as well as sent over GNUstep's Distributed objects mechanism.
See the reference manual for more details.