Is the callback functionality of SocketIO a feature exclusive to SocketIO? And if so, how is the header formed?

Is the callback functionality of SocketIO a feature exclusive to SocketIO?
And if so, how is the header formed?

I'm creating a native mobile project that will use SocketIO on NodeJS as
the communications API.
We would like to use the callback functionality of SocketIO, but on native
iOS (or Android).
However, the callback functionality doesn't appear in any client
implementation of WebSockets that I've found so I am guessing it is a
feature exclusive to SocketIO?
If so, what is the header information before the payload that socketIO
uses to make this possible.
Over the wire I can see some binary before the JSON payload, but I'm not
enough of a JS guru to analyse how the callback functionality on the
client side is done, so that I can replicate that in ObjC and Java.