There are two common types of Tank Mounted Fuel Valve Petcocks or a Remote Mounted Fuel Valve Petcock for a gravity feed.  Gravity feed is for when the Tank is mounted vertically higher than the carburetor.  When the tank is lower than the carburetor there would be fuel pump with same hose connections.  All are actuated by a vacuum line coming off of the intake manifold to facilitate fuel to flow through the fuel line to the carburetor.

Often times when the intake manifold does not have enough ports the vacuum line is split off with a T, Y or X to power the petcock or fuel pump, the carburetor and other items like emission controls.

Here is the typical hose routing on GY6 fuel and vacuum lines.