Javascript/Ember Syntax '%@'

Anyone know what this does or what this is? It looks very strange to me. In the context of Ember.js, it is being used in the REST adapter like so:

nested/%@/model, almost like a wildcard for the model type maybe?

Here is a URL where it is referenced: http://ift.tt/1MqpPQf

I can't seem to find anything in the documentation about it, but have seen it used more than once.