The bus tool draws a thick line to denote a bus.
The bus indicates
that multiple wire connections are routed from source pins to
destination pins. Buses can be labeled with a netname
using the netnaming tool and the netname will be dragged with the bus
as it is moved.
Note however that the bus is purely a visual decoration and has no relevance to the netlister. It is provided only to improve the readability of a schematic. As a consequence the netname is also purely descriptive and also plays no part in netlisting.
The netlist is only defined by pins and netnames attached to wires.
The thickness of the bus is controlled from Edit-->Options-->Schematic-->Bus width.