The iteration mode is set outside the flow because the quantities for Input and Output cannot make sense in other modes. For example if you are in Vertex mode, it does not make sense to read or write to UVW channels since those are quantities stored relative to the corners of a face and not on the vertex. Since there are potentially many texture verts that might occupy the same physical location as a geometric vertex there is no way to write to a texture vert when your Genome flow is looping over all the geometric vertices. The same logic applies when you are iterating over the faces of a mesh. What would reading or writing to the the Position channel actually do when all you know is you are working on face X?
Think about Genome this way:
In VERTEX mode:
for i = 1 to (getNumVerts theMesh) do (
doSomethingWithThisVertex theMesh i
)
In FACE mode:
for i = 1 to (getNumFaces theMesh) do (
doSomethingWithThisFace theMesh i
)
In CUSTOM FACE VERTEX mode:
for i = 1 to (getNumFaces theMesh) do (
for j = 1 to 3 do (
doSomethingWithThisFaceCorner theMesh i j
)
)