The Sample Pos. to UV projection type converts the screen space position of the current sample to the UVW coordinate (figure 1). This projection type is designed to be used with the Global Texture AOV. This projection is simple and can be used to create masks for compositing on-the-fly. This is not the same as screen space projection, which maps the sample position in world space into screen space.
Figure 1: Projecting a Check texture using the Sample Pos. to UV node as the projection type