Woodnthings, the pictures show two completely different things. The straight edge against the blade is used for aligning the table saw top with the miter ways. This only needs to be done once in many cases but should be checked from time to time, especially on contractor and other portable type saws. It's one part of tuning a table saw.
The second set of pictures in the other post is how I set my rip fence when I want to rip a board, on a tuned saw. As I said, I've had many rip fences on different table saws. Most of the newer ones, Unifence, Biesmeyer, etc. also have a scale, even my old jet lock has one but I don't use them. Blades vary in thickness and I often add an auxilliary fence, both of which throw off the scale. Instead, I just hold my measuring tape at the measurement I want to cut, 8" in the picture, and then look to see what measurement the tape reads at the edge of the miter way. Then I move my fence over to that measurement from the miter way. Then I lock down the fence and check the outfeed end. In other words, I don't slide the fence over measuring to the blade tooth like so many do and I did. I want the rip fence strong on the outfeed end just the thickness of the mark on my measuring tape and during my tune up process above, set the fence so, but I still never trust it, ever. The slight amount I off set the outfeed end is why I tried to show the two sides of the mark on the measuring tape with the awl as a pointer.
My Unfence is usually dead on, as I adjusted it when tuning, but I never take for granted that it is. I always check both ends to the miter way every time I set the fence for a rip. I didn't used to but once in a while it wouldn't be correct and I might get burning or excess marking on the cut or the board might pull away from the blade, depending on which direction the fence was off. My old habit of just setting the pointer to the fence scale, locking down the fence and going to work is over. Some folks may think I'm nuts but I crosscut a lot of plywood using my rip fence, often pieces that are a lot longer than they are wide. You can be in deep trouble with this type of cut if the fence isn't correct and I don't recommend this for folks that are not in command of their table saws. Those of us that do it every day have no trouble handling large sheets or making cuts that others wouldn't consider doing on a table saw. When you walk that fine edge bordering on kickback, you make sure the saw and fence are set up properly but I want it correct for every cut, every time.
Since I do this for a living, I approach my tools like a truck driver starts a haul. Before I start up, I check all the tires, lights, signals and on through the check list. There are still dangers to be faced but at least I know the equipment is ready. I never get in my car or truck without taking a walk around. In the business, it's part of your training and it becomes second nature. I try not to forget a single accident can end your career. Most folks get hurt on woodworking equipment from not taking an extra second or two. We usually ruin a few work pieces for the same reason. Either one costs me and I can't afford it.