VertexCoordinateRules is an option for GraphPlot and related functions which specifies rules for determining the coordinates at which vertices should be placed.
VertexCoordinateRules specifies explicit coordinates for vertices with names , , ....
Vertices whose coordinates are not explicitly specified in the rules given by VertexCoordinateRules are automatically assigned coordinates by the graph layout procedure.
A setting Automatic specifies that the vertex with name should be assigned coordinates automatically.
vk->{x, Automatic} defines an explicit x coordinate, but specifies that the y coordinate should be assigned automatically. vk->{Automatic, y} defines instead an explicit y coordinate.