The spec used to talk about MQTT Clients. It now talks about MQTT Publishers and MQTT Subscribers - as if they are completely different things. But ultimately, an MQTT Publisher can subscribe to things and an MQTT Subscriber can publish to things.

I'm not sure it is helpful to introduce the idea they are different entities when they are not. It implies you need two different clients if you want to do both pub and sub - a misapprehension we've had to respond to on the mailing list in the past.