Coverage mask AA techniques can process samples in a pixel with the same precision as individual point samples if the information is kept around to do so. This includes the correct processing of implicit edges. Z3 does this by using z slopes (hence Z3 - the z, dzdx, and dzdy), although there are other techniques, this is the most effective. FAA did not bother to do this.
When using z slopes with coverage mask AA, the result is actually a form of tile based renderer with pixel sized tiles. The rendering is done in two passes, much like a typical TBR. The first pass renders to "tiles" that are a rectangular collection of higher resolution screen samples. The second pass then renders each tile to the final frame buffer.
The major differences compared to typical TBRs are the format of the data in the tile and the size of the tile. In the case of coverage mask AA, the data is kept in the tile as a set of geometry information (z slopes and a z value), bit masks, and colors. This means that colors are rendered on the first pass which is different than a deferred rendering tiler which keeps the geometry as triangles and unrendered color information. However, both render at a lower resolution than the sample resolution on the first pass and collect up fragments in the "tile" buffer to be rendered on the second pass. On the second pass coverage mask AA, processes each pixel sized "tile" by processing each of the fragments found on the first pass - typically front to back to produce the final color for the pixel.
A typical TBR of course, processes each tile by processing each of the triangles in the tile (not necessarily front to back).
Coverage mask AA derives many of the same benefits as typical TBRs, that is, it substantially reduces external memory bandwidth for a given sample resolution. This is because the processing on the first pass is done at a lower resolution than the sample resolution, and the results are kept in a compressed format for the second pass which processes each tile.
There is one notable difference when it comes to AA though between coverage mask techniques and typical TBRs. Typical TBRs are capable of rendering super-sampled AA at high sample densities (say 16x) while coverage mask techniques must use multi-sampling. This is because TBR's do not render the color on the first pass, but only when the tile is rendered. Since coverage mask AA renders colors on the first pass, it can only afford to render one color per fragment, since rendering a color for each sample would lose most of the benefits.
This may not be much of an advantage in actual practice though, since a TBR would be limited by pixel shader performance, even though it is not restricted by external memory bandwidth. Super-sampling each sample when running a sophisticated pixel shader is simply impractical. The computational resources could be put to much better use and multi-sampled AA is good enough.
