Thinset is a lot more complex that many people think! While a basic thinset is always made up of cement and an aggregate (often, a fine sand), there are many different chemical compounds that are called cement, lots of different aggregates, and their preparation and capabilities can vary not only by the type of cement, but by the aggregates and the proportions of each. To help make some sense out of these, we have ANSI standards, but they are meaningless unless you have at least a cursory understanding of what they mean. Right now, there are four different major groupings of thinsets for the USA customer: ANSI A118.1, A118.4, A118.11, and a new one A118.15. Then, those can have an extra character on them such as E or F where 'E' stands for extended working time (takes a little longer to start to set up) and 'F' stands for faster setting (often referred to as rapid set). Keep in mind that as the .X part of the specification increases, it also has the properties of the lower numbered specification as well. So, an A118.11 will have at least all of the specifications of an A118.4 mortar. Also note, that the specifications are a MINIMUM, and often, within a particular specification, there can be an economy version and a premium version, and, it is possible that a lower numbered specification may exceed at least some of the properties of a higher numbered specifications, but not all of them, so according to the specification, you cannot call it that higher numbered material. There are some thinset manufacturers out there that are saying that they meet the A118.15 specification, but when you read the fine print, it may only be in one or several key characteristics. This is against the 'rules' of the ANSI organization, so beware...if you actually need a mortar that meets a particular spec in ALL capabilities (often, that is the goal, especially if you are a professional where the job specification calls for a particular thing), make sure to read the fine print or follow the asterisk or whatever and read the fine print. So, now let's look at the basic characteristics of the different specifications. When it comes to A118.4, .11, and .15; they are all referred to as modified mortars. This means that they have some additional admixture in them to make them more flexible and other characteristics, which can include freeze/thaw, hot/cold, impact, shear strength, and a bunch more 'features'. For a rough indication, you could call these good, better, best in the modified thinset mortar groups. Keep in mind that any particular job may never stress the products involved, and good may be much more than is required. Then, there may be a job where best is the only one that might survive under the conditions. It is somewhat foolish to always go for best when it won't make any difference...sort of like using premium gasoline in your car when it is designed to run on regular. What I haven't touched on is A118.1 mortars. These are not a modified mortar (meaning it does not have an admix in it), and are sometimes called an unmodified or dryset mortar. Depending on the products used, and the manufacturer's recommendations, this may be much more than adequate, and often is chosen for some very important reasons. A quick word on admixes. Often, these are mixed into the cement and aggregate mixture in the factory as dry particles. They can be made from laytex, acrylic, and other materials. They each have their unique characteristics. Every manufacturer of a dryset mortar (A118.1) also sells a liquid admix that can be used to create a modified mortar. Depending on how much of the stuff and whose (do not mix manufacturer's products or you'll get unreliable results!) you may be able to turn that A118.1 mortar into one meeting one of the A118.4 or A118.11 types (have not seen one that can make a A118.15 with admix, but it may be out there). A final word, each of the manufacturer's includes some fairly specific instructions on how to mix their products such as the amount of liquid to add, how long to mix it, at what speed to mix it, and how long to let it slake before using it. If you do not follow those instructions, you will NOT meet the whole specification for what you bought.