Hi Andrei, yeah like Danny writes above (thanks Danny!) it is an option for the projective mode of Flow.

For the default (non-projective) mode of Flow, things work more like texture mapping and the particular location of the base plane doesn't matter. In this mode things work more like UV texture mapping.

In Projective mode the location of the base plane does matter and you need to position the base plane above the target object in such a way that the normals of the base plane will shoot out and intersect the target object, that's the projection part.

Then by default the deformation will track off the surface normal of the target object working like surface offsetting. The "Straight" option makes the deformation track along the projection direction instead giving the extrude-like shape rather than the offset like shape.

In non-projective flow mode, it's assumed that you may be wrapping things all the way around a cylinder or something like that, where it would just not make sense to do movement in one single fixed direction. With projective mode it's assumed that you are placing the object onto a local area of the target object that does not twist all the way around to the opposite direction.

Hope that makes sense.

- Michael