Using the Flatten tool you can flatten a single or multiple Polygons and you can flatten in 2 modes: Along Edge or Along Normal.
Flatten can take 2 kinds of selections.
You can either select 3 vertices or multiple polygons* with one being active**.
If you select 3 vertices, all verts of the connected polygon will be flattend based on the normal of the implicit triangle the 3 verts create.
If you select 2 or poly polygons, all polygons will be flattened based on the normal of the active (last selected) polygon.
3 vert selection and 2+ polygon selection