Each node in the circuit requires a unique name. You can specify the name of a node so an arbitrary one isn't generated by the netlister. Node "0" is the circuit global ground and is drawn with a special graphical symbol instead of the name "0".
There is also a graphical symbol defined for node "COM", but this node has no special significance. That is, it's not the SPICE global common and it's not even a global node. It's just sometimes convenient to have a graphical symbol associated with a node distinct from ground.
If you give a node a name starting with the characters "$G_"; as in for example, "$G_VDD"; then that node is global no matter where the name occurs in the circuit hierarchy.
It is possible to indicate that a node is a port of type input, output, or bi-directional. These port types will be drawn differently but have no significance to the netlister. Indicating a port type can make circuit more readable. Global nodes are also drawn differently in that a box is drawn around the name.
