Modular Rig Assembly
The biggest difference between ACS2 and ACS3 (from user point of view) will probably be the fact that ACS3 is a fully modular rigging system. Modules in ACS3 are self contained building blocks from which more complex rigs can be assembled.
Each module needs to have at least one Plug and one Socket for it to be connectable to other modules. Plugs are points with which a module can connect to another module while Sockets are points at which other modules can connect to a module that exposes a Socket. There might be other connecting points between modules (this is still under development) but these will be handled automatically based on the plug to socket connections that user performs manually. In short - all that is required from you as a user to make a connection between modules is to insert a Plug of one module to the Socket of another one. Anything else will be handled by ACS3 automatically.
ACS3 modules are (pretty much) standard MODO assemblies and are stored in standard assembly preset format (.lxp). It means that they show up and can be dragged and dropped into the scene directly from Preset Browser. The workflow for assembling modules together is also based on drag and drop actions.
See video below for a presentation of the aforementioned workflows at the current stage of ACS3 development. Feel free to ask any questions of provide feedback as well, it will help in making this tool better :).