Always read the active ingredient percentage when buying chlorine. There’s a reason some are cheaper.
There are several types of chlorine shock and tablets, but the one commonality between them is that they all produce hypochlorous acid to sanitize the water. When dealing with a granular shock, I prefer to use calcium hypochlorite. This shock is unstabilized, which means that it does not contain any cyanuric acid and it does not increase the stabilizer level of the pool water. Calcium hypochlorite will increase the pH a little and it does contain calcium, so your calcium level will rise some. Second on my list, we have sodium-dichloro-s-triazinetrione, or “dichlor”, as most people call it. Dichlor shock contains cyanuric acid or stabilizer. Stabilizer is good to have in the right amount, but several times each year I encounter a pool where the stabilizer level is out of control. Once it becomes too high, the water is cloudy, slightly green, and the only solution is to drain a significant portion of the water and refill. The problem is that most chlorine tablets are trichlor, which is stabilized as well, and then you are also adding a dichlor shock. This is why I recommend the calcium hypochlorite (cal-hypo) shock. One exception to this might be with a new vinyl liner. Dichlor shock is less likely to bleach out a vinyl liner. I have already mentioned trichlor or trichloro-s-triazinetrione. As stated, most tablets are in this form.
This is by no means a complete and thorough chlorine lesson. What I would like for you to take from this is that there are different types of chlorine and each type has a specific purpose. I also want you to realize that you are adding more than just chlorine to the water and it is important to know what you are adding and what the levels are at for that extra chemical that is being added.