Specifically, "reduce tiles" lowers the number of tiles it takes to make a full sprite, but it comes at the potential cost of using more sprite pieces (you only get 80 total in one state to work with if the horizontal resolution is set to 320px, also only 20 sprite pieces per scanline, each piecing ranging from 1x1 tiles to 4x4 tiles). It really depends on what you need, but for a standard Sonic hack, you'll more than likely be fine with tile reduction. Basically, you'd pick based on if you prioritize lower VRAM usage or a lower sprite count. Say, I have this sprite here: In tile reduction mode, it'd do something like this to make up the sprite: 4 sprite pieces, but only 6 tiles. Since it's able to be split up like that, tiles can be flipped, and thus increase the tile savings. Another potential scenario in tile reduction mode is this: Compared to the scenario above, sprite usage is cut in half by adding in 2 blank tiles so that the 1x2 and 1x4 pieces can be merged into 1 2x4 piece. Mirroring is still applied to save on tile usage. Which of the 2 scenarios above Flex 2 does, to be honest, I have not a clue; I don't really use Flex 2 all that often and have not paid that much attention. However, in mappings reduction mode, I know that it'd do this: Only 1 sprite piece, but 16 tiles! Since it's not split into 2 sprite pieces, it's impossible to apply the mirroring trick. It's kind of a pick your poison scenario. Choose what you think would be most appropriate to use.