What are the two types of PBX?

Could you please elaborate on the two distinct types of Private Branch Exchange (PBX) systems that exist? I'm particularly interested in understanding the key differences between them, as well as their respective advantages and limitations in terms of functionality, scalability, and cost-effectiveness. Additionally, how do these two types of PBX systems compare in terms of their compatibility with modern communication technologies and their ability to support remote and mobile workforces?